Remove shebangs from various setup.py files (#5867)

setup.py files are not executable, and thus have no need for shebangs.

Also remove unnecessary comments

src/sonic-bgpcfgd/setup.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-
 import setuptools
 
 setuptools.setup(

src/sonic-yang-mgmt/setup.py

@@ -1,8 +1,3 @@
-#!/usr/bin/env python
-# -*- coding: utf-8 -*-
-
-"""The setup script."""
-
 from setuptools import setup, find_packages
 
 # read me

src/sonic-yang-models/setup.py

@@ -1,8 +1,3 @@
-#!/usr/bin/env python
-# -*- coding: utf-8 -*-
-
-"""The setup script."""
-
 from setuptools import setup, find_packages
 
 # read me