Remove CustomUserMessageAuthenticationException

Remove the use of `CustomUserMessageAuthenticationException`, as this is not compatible with the `@throws` tag of the interface.
Since `AccountDeletedException` is implemented by the user here, they could let it take a custom message.

Author: rossdeane

security/user_checkers.rst

@@ -38,11 +38,6 @@ are not met, an exception should be thrown which extends the
             // user is deleted, show a generic Account Not Found message.
             if ($user->isDeleted()) {
                 throw new AccountDeletedException('...');
-
-                // or to customize the message shown
-                throw new CustomUserMessageAuthenticationException(
-                    'Your account was deleted. Sorry about that!'
-                );
             }
         }