Made servers modified via cvar changes appear in normal servers

In an attempt to address #2856 I made these changes which make servers which are running the mod "xonotic" (which is the default value) appear in the normal servers category.

Implementation makes the cvar menu_slist_purethreshold unused which used to be a "allow this many impure cvars before making the server appear in modified, it was removed.

Examples:

Modified cvars server running the base game:

Modified servers category is still there:

Perhaps in a follow-up MR we could add a grayed out Xonotic logo icon for modified Xonotic servers? This would fill most of the empty slots for that icon in the serverlist and still convey the information that they have some impure cvars.