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/api/oidc/integration_test/token_exchange_test.go

@@ -52,13 +52,6 @@ func setTokenExchangeFeature(t *testing.T, instance *integration.Instance, value
 	time.Sleep(time.Second)
 }
 
-func resetFeatures(t *testing.T, instance *integration.Instance) {
-	iamCTX := instance.WithAuthorization(CTX, integration.UserTypeIAMOwner)
-	_, err := instance.Client.FeatureV2.ResetInstanceFeatures(iamCTX, &feature.ResetInstanceFeaturesRequest{})
-	require.NoError(t, err)
-	time.Sleep(time.Second)
-}
-
 func setImpersonationPolicy(t *testing.T, instance *integration.Instance, value bool) {
 	iamCTX := instance.WithAuthorization(CTX, integration.UserTypeIAMOwner)