Remove legacy healthcheck files and structures

[frouioui]

Description

This Pull Request removes the legacy healthcheck and the related code (legacy topo watcher, legacy replication lag, etc). The different usages have been replaced by the new implementation of the healthcheck. The different files hosting the legacy code have been removed.

Related Issue(s)

• Part of Task: delete deprecated discoverygateway and legacy healthcheck code #9218
• Closes Delete legacy_healthcheck.go and any related structs that are no longer needed #10454

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

[github-actions]

Review Checklist

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[deepthi]

Since this is cleanup, we don't need to back port it to v14. It's a teeny bit risky to refactor this close to release on the release branch.

[deepthi]

Why are we embedding an interface into another? Do we have implementations of TabletRecorder that don't implement HealthCheck?

[frouioui]

Do we have implementations of TabletRecorder that don't implement HealthCheck?

No, we don't.

Why are we embedding an interface into another?

This was done so we can use the healthcheck (discovery.HealthCheck) as a tablet recorder in tx throttler:

vitess/go/vt/vttablet/tabletserver/txthrottler/tx_throttler.go

Line 199 in 4654a77

topologyWatcherFactory = func(topoServer *topo.Server, tr discovery.TabletRecorder, cell, keyspace, shard string, refreshInterval time.Duration, topoReadConcurrency int) TopologyWatcherInterface {

Ultimately, the topo watcher in the discovery package uses a TabletRecorder.

Previously, the LegacyHealthCheck interface was embedding LegacyTabletRecorder:

vitess/go/vt/discovery/legacy_healthcheck.go

Lines 285 to 289 in d037f24

	type LegacyHealthCheck interface {
	// LegacyTabletRecorder interface adds AddTablet and RemoveTablet methods.
	// AddTablet adds the tablet, and starts health check on it.
	// RemoveTablet removes the tablet, and stops its StreamHealth RPC.
	LegacyTabletRecorder

[deepthi]

we should not have to embed it for things to work.

[deepthi]

Only one change is required - to remove TabletRecorder from HealthCheck interface.
Once that is done, this can be merged.
Nice work, and thank you for your patience during review.

[frouioui]

Only one change is required - to remove TabletRecorder from HealthCheck interface.

We decided to remove the TabletRecorder interface and move its methods directly to the HealthCheck interface. They were both implemented by the same set of structures. This was done via b89882f.