Don't mix in user options of printer instances

GTK+ currently does not support printer instances in the printing
dialog. This commit avoids user options set for specific instances of
a printer being accidently applied to the printer with the same name but
no specific instance defined.

https://bugzilla.gnome.org/show_bug.cgi?id=753628
https://bugzilla.gnome.org/show_bug.cgi?id=582747

modules/printbackends/cups/gtkprintbackendcups.c

@@ -3967,12 +3967,9 @@ cups_parse_user_options (const char     *filename,
       if (!*lineptr)
         continue;
 
-      /* NUL-terminate the name, stripping the instance name */
       name = lineptr;
       while (!isspace (*lineptr) && *lineptr)
         {
-          if (*lineptr == '/')
-            *lineptr = '\0';
           lineptr++;
         }