Remove Python3 venv from sonic-mgmt-docker build file (#18130)

Why I did it
This is sonic-mgmt-docker Python3 migration.

How I did it
Remove Python3 venv from sonic-mgmt-docker

How to verify it
Compile docker and verify the Python3 venv is NOT there.

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -265,91 +265,6 @@ WORKDIR /var/$user
 # Add az symlink for backwards compatibility
 RUN mkdir bin && ln -s /usr/bin/az bin/az
 
-{% if legacy == 'y' or legacy == '1' %}
-RUN python3 -m venv --system-site-packages env-python3
-
-# Activating a virtualenv. The virtualenv automatically works for RUN, ENV and CMD.
-ENV VIRTUAL_ENV=env-python3
-ARG BACKUP_OF_PATH="$PATH"
-ENV PATH="$VIRTUAL_ENV/bin:$PATH"
-
-ENV LANG=C.UTF-8 LC_ALL=C.UTF-8 PYTHONIOENCODING=UTF-8
-
-RUN python3 -m pip install --upgrade pip setuptools wheel
-RUN python3 -m pip install aiohttp \
-                           allure-pytest==2.8.22 \
-                           ansible==2.9.27 \
-                           azure-storage-blob==12.9.0 \
-                           azure-kusto-data \
-                           azure-kusto-ingest \
-                           defusedxml \
-                           celery[redis]==5.2.7 \
-                           cffi \
-                           contextlib2==0.6.0.post1 \
-                           cryptography==41.0.2 \
-                           ctypesgen \
-                           dpkt \
-                           dpugen==0.1.1 \
-                           "future>=0.16.0" \
-                           gitpython \
-                           ipaddr \
-                           ipython==8.12.2 \
-                           ixload \
-                           ixnetwork-restpy==1.0.64 \
-                           ixnetwork-open-traffic-generator==0.0.79 \
-                           jinja2==2.10.1 \
-                           jsonpatch \
-                           lxml \
-                           markupsafe==2.0.1 \
-                           mock \
-                           msrest==0.6.21 \
-                           natsort \
-                           ncclient \
-                           netaddr \
-                           netmiko==2.4.2 \
-                           nnpy \
-                           pandas \
-                           paramiko==2.7.2 \
-                           passlib \
-                           pexpect \
-                           prettytable \
-                           psutil \
-                           ptf \
-                           pyasn1==0.4.8 \
-                           pycryptodome==3.9.8 \
-                           pyfiglet \
-                           pylint==1.8.1 \
-                           pyro4 \
-                           pysnmp==4.4.12 \
-                           pysubnettree \
-                           pytest-ansible \
-                           pytest-html \
-                           pytest-repeat \
-                           pytest-xdist==1.28.0 \
-                           python-dateutil \
-                           pytest==7.1.3 \
-                           PyYAML \
-                           redis \
-                           requests \
-                           retry \
-                           rpyc \
-                           scandir \
-                           scapy==2.5.0 \
-                           setuptools-rust \
-                           six \
-                           snappi==0.9.1 \
-                           snappi-convergence==0.4.1 \
-                           snappi-ixnetwork==0.9.1 \
-                           sshconf==0.2.5 \
-                           tabulate \
-                           textfsm==1.1.2 \
-                           thrift==0.11.0 \
-                           virtualenv
-
-# Deactivating a virtualenv
-ENV PATH="$BACKUP_OF_PATH"
-{% endif %}
-
 USER root
 WORKDIR /azp
 COPY start.sh \