[dhcp_relay] Fix import for dhcp_counters on clear_dhcp6relay_counter.py (#8991)

#### Why I did it

**Import issue will cause:**
root@sonic:/# sudo sonic-clear arp
failed to import plugin clear.plugins.dhcprelay: No module named 'show_dhcp_relay'

#### How I did it

Fix the import.

#### How to verify it

run sudo sonic-clear arp

dockers/docker-dhcp-relay/cli/clear/plugins/clear_dhcp6relay_counter.py

@@ -1,7 +1,6 @@
 import sys
 import click
-sys.path.insert(0, '../../show/plugins/')
-from show_dhcp6relay_counters import DHCPv6_Counter
+from show.plugins.dhcprelay import DHCPv6_Counter
 
 import utilities_common.cli as clicommon
 
@@ -15,7 +14,7 @@ def dhcp6relay_clear():
 @click.option('-i', '--interface', required=False)
 def dhcp6relay_clear_counters(interface):
     """ Clear dhcp6relay message counts """
-    
+
     counter = DHCPv6_Counter()
     counter_intf = counter.get_interface()