Revert "Fix the build with cups < 2.3"

This reverts commit d761e3cf2c.

I am seeing PPD_CUSTOM_UNKNOWN in the cups headers in our
ci images, and that is cups 2.2.12. So this commit was
mistaken.

modules/printbackends/gtkprintbackendcups.c

@@ -647,9 +647,7 @@ add_cups_options (const char *key,
                           custom_value = TRUE;
                           break;
 
-#if CUPS_VERSION_MAJOR == 2 && CUPS_VERSION_MINOR >= 3
                         case PPD_CUSTOM_UNKNOWN:
-#endif
                         default :
                           custom_value = FALSE;
                         }