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/feature/feature_test.go

@@ -12,7 +12,6 @@ func TestKey(t *testing.T) {
 		"unspecified",
 		"login_default_org",
 		"trigger_introspection_projections",
-		"legacy_introspection",
 	}
 	for _, want := range tests {
 		t.Run(want, func(t *testing.T) {