Remove dead code identified by deadcode tool (#37271)

Ran [`deadcode`](https://pkg.go.dev/golang.org/x/tools/cmd/deadcode)
(`-test ./...`) to find functions, methods and error types unreachable
from any call path (including tests), and removed the truly-dead ones.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

models/asymkey/error.go

@@ -192,28 +192,6 @@ func (err ErrGPGKeyIDAlreadyUsed) Unwrap() error {
 	return util.ErrAlreadyExist
 }
 
-// ErrGPGKeyAccessDenied represents a "GPGKeyAccessDenied" kind of Error.
-type ErrGPGKeyAccessDenied struct {
-	UserID int64
-	KeyID  int64
-}
-
-// IsErrGPGKeyAccessDenied checks if an error is a ErrGPGKeyAccessDenied.
-func IsErrGPGKeyAccessDenied(err error) bool {
-	_, ok := err.(ErrGPGKeyAccessDenied)
-	return ok
-}
-
-// Error pretty-prints an error of type ErrGPGKeyAccessDenied.
-func (err ErrGPGKeyAccessDenied) Error() string {
-	return fmt.Sprintf("user does not have access to the key [user_id: %d, key_id: %d]",
-		err.UserID, err.KeyID)
-}
-
-func (err ErrGPGKeyAccessDenied) Unwrap() error {
-	return util.ErrPermissionDenied
-}
-
 // ErrKeyAccessDenied represents a "KeyAccessDenied" kind of error.
 type ErrKeyAccessDenied struct {
 	UserID int64