Remove gbsyncd_start.sh (sonic-net#808)

`gbsyncd_start.sh` was a simple wrapper which only called `exec "/usr/bin/gbsyncd_startup.py"`. This causes unnecessary extra complexity and maintenance. It makes things simpler to run `/usr/bin/gbsyncd_startup.py` directly, which is being done by sonic-net#7084. That PR has merged, so we can now merge this.

Author: jleveque