Task: delete deprecated discoverygateway and legacy healthcheck code

[deepthi]

As part of #5750, we implemented a new gateway called tabletgateway and deprecated the old one called discoverygateway.
We have had several releases since then, and tabletgateway is now stable enough. We can now start deleting some of the old code.

Blocker: vtctld ui still uses legacy healthcheck. We need to either move that to the new healthcheck, or deprecate/delete that first.

Tasks

• In release 13.0, deprecate and ignore -gateway_implementation flag Deprecate and ignore gateway_implementation flag to vtgate. #9482
• delete discoverygateway.go Delete discovery gateway #9500
• Optionally also delete Gateway interface Remove the Gateway interface #9852
• fix vtctld realtime_status to not use legacy healthcheck Task: change vtctld's realtime_status to use new health check #9358
• delete legacy_healthcheck.go and any related structs that are no longer needed Delete legacy_healthcheck.go and any related structs that are no longer needed #10454
• In release 14.0 delete -gateway_implementation flag Remove the Gateway interface #9852

EDIT Mar 8, 2022:
Split deletion into two separate tasks because of the dependency on #9358

[deepthi]

All tasks are now complete. Thank you for all your work on this @frouioui!