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/client_credentials.go

@@ -14,27 +14,6 @@ import (
 	"github.com/zitadel/zitadel/internal/zerrors"
 )
 
-type clientCredentialsRequest struct {
-	sub      string
-	audience []string
-	scopes   []string
-}
-
-// GetSubject returns the subject for token to be created because of the client credentials request
-// the subject will be the id of the service user
-func (c *clientCredentialsRequest) GetSubject() string {
-	return c.sub
-}
-
-// GetAudience returns the audience for token to be created because of the client credentials request
-func (c *clientCredentialsRequest) GetAudience() []string {
-	return c.audience
-}
-
-func (c *clientCredentialsRequest) GetScopes() []string {
-	return c.scopes
-}
-
 func (s *Server) clientCredentialsAuth(ctx context.Context, clientID, clientSecret string) (op.Client, error) {
 	user, err := s.query.GetUserByLoginName(ctx, false, clientID)
 	if zerrors.IsNotFound(err) {