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/command/system_features.go

@@ -12,7 +12,6 @@ import (
 type SystemFeatures struct {
 	LoginDefaultOrg                 *bool
 	TriggerIntrospectionProjections *bool
-	LegacyIntrospection             *bool
 	TokenExchange                   *bool
 	UserSchema                      *bool
 	ImprovedPerformance             []feature.ImprovedPerformanceType
@@ -26,7 +25,6 @@ type SystemFeatures struct {
 func (m *SystemFeatures) isEmpty() bool {
 	return m.LoginDefaultOrg == nil &&
 		m.TriggerIntrospectionProjections == nil &&
-		m.LegacyIntrospection == nil &&
 		m.UserSchema == nil &&
 		m.TokenExchange == nil &&
 		// nil check to allow unset improvements