Remove obsolete error constants

- ERROR_RELATIONSHIP_ALREADY_EXISTS
  see issue #22363 commit 1516e6e
- ERROR_SESSION_HANDLER_INVALID
  see issue #23966 commit 1ed490a

These have effectively not been used in the code for years, but had
to be kept until they were no longer referenced in language files.

api/soap/mc_api.php

@@ -231,7 +231,6 @@ static function faultFromException( Exception $p_exception ) {
 			case ERROR_SPONSORSHIP_NOT_ENABLED:
 			case ERROR_SPONSORSHIP_AMOUNT_TOO_LOW:
 			case ERROR_SPONSORSHIP_SPONSOR_NO_EMAIL:
-			case ERROR_RELATIONSHIP_ALREADY_EXISTS:
 			case ERROR_RELATIONSHIP_SAME_BUG:
 			case ERROR_LOST_PASSWORD_CONFIRM_HASH_INVALID:
 			case ERROR_LOST_PASSWORD_NO_EMAIL_SPECIFIED:

core/constant_inc.php

@@ -384,7 +384,6 @@
 define( 'ERROR_SPONSORSHIP_SPONSOR_NO_EMAIL', 1705 );
 
 # ERROR RELATIONSHIP
-define( 'ERROR_RELATIONSHIP_ALREADY_EXISTS', 1800 );  # TODO: remove once no longer used in lang files
 define( 'ERROR_RELATIONSHIP_ACCESS_LEVEL_TO_DEST_BUG_TOO_LOW', 1801 );
 define( 'ERROR_RELATIONSHIP_NOT_FOUND', 1802 );
 define( 'ERROR_RELATIONSHIP_SAME_BUG', 1803 );
@@ -433,10 +432,6 @@
 define( 'ERROR_COLUMNS_INVALID', 2601 );
 
 # ERROR_SESSION_*
-define( 'ERROR_SESSION_HANDLER_INVALID', 2700 );
-# ERROR_SESSION_HANDLER_INVALID is no longer used in code
-# but can't be removed as long as it's used in one of the language files
-# for $MANTIS_ERROR[ERROR_SESSION_HANDLER_INVALID]
 define( 'ERROR_SESSION_VAR_NOT_FOUND', 2701 );
 define( 'ERROR_SESSION_NOT_VALID', 2702 );