[config][loopback] Fix Bug that cannot add loopback interface by CLI (sonic-net#1276)

**- What I did**
Fix that it cannot add loopback interface by CLI, like the following:
```
root@as7816-64x:~# config loopback add Loopback1
Usage: config [OPTIONS] COMMAND [ARGS]...
Try "config --help" for help.

Error: No such command "L".
```
**- How I did it**
Remove `@config.group()` before the function `is_loopback_name_valid(loopback_name)`, which will be called by add/del command

Signed-off-by: MuLin <[email protected]>

Author: MuLinForest

config/main.py

@@ -3188,7 +3188,6 @@ def naming_mode_alias():
     """Set CLI interface naming mode to ALIAS (Vendor port alias)"""
     set_interface_naming_mode('alias')
 
-@config.group()
 def is_loopback_name_valid(loopback_name):
     """Loopback name validation
     """