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/system_features.go

@@ -22,7 +22,6 @@ type SystemFeatures struct {
 
 	LoginDefaultOrg                 FeatureSource[bool]
 	TriggerIntrospectionProjections FeatureSource[bool]
-	LegacyIntrospection             FeatureSource[bool]
 	UserSchema                      FeatureSource[bool]
 	TokenExchange                   FeatureSource[bool]
 	ImprovedPerformance             FeatureSource[[]feature.ImprovedPerformanceType]