chore(oidc): remove legacy storage methods (#10061)

# Which Problems Are Solved

Stabilize the optimized introspection code and cleanup unused code.

# How the Problems Are Solved

- `oidc_legacy_introspection` feature flag is removed and reserved.
- `OPStorage` which are no longer needed have their bodies removed.
- The method definitions need to remain in place so the interface
remains implemented.
  - A panic is thrown in case any such method is still called

# Additional Changes

- A number of `OPStorage` methods related to token creation were already
unused. These are also cleaned up.

# Additional Context

- Closes #10027 
- #7822

---------

Co-authored-by: Livio Spring <livio.a@gmail.com>

internal/query/projection/system_features.go

@@ -60,10 +60,6 @@ func (*systemFeatureProjection) Reducers() []handler.AggregateReducer {
 				Event:  feature_v2.SystemTriggerIntrospectionProjectionsEventType,
 				Reduce: reduceSystemSetFeature[bool],
 			},
-			{
-				Event:  feature_v2.SystemLegacyIntrospectionEventType,
-				Reduce: reduceSystemSetFeature[bool],
-			},
 			{
 				Event:  feature_v2.SystemUserSchemaEventType,
 				Reduce: reduceSystemSetFeature[bool],