[docker-gbsyncd-vs] Run gbsyncd_startup.py directly (#7084)

Eliminate the need for `gbsyncd_start.sh`, which simply calls `exec "/usr/bin/gbsyncd_startup.py"`. The shell script is unnecessary.

Once this PR merges, we can remove `gbsyncd_start.sh` from the sonic-sairedis repo.

platform/vs/docker-gbsyncd-vs/supervisord.conf

@@ -38,7 +38,7 @@ dependent_startup=true
 dependent_startup_wait_for=rsyslogd:running
 
 [program:syncd]
-command=/usr/bin/gbsyncd_start.sh
+command=/usr/bin/gbsyncd_startup.py
 priority=3
 autostart=false
 autorestart=false