Clean up some more "favourite profile"-related code

- Since the favourite profile feature was removed in commit 28ba920c,
there is no need to sort profiles by menu index, this simlifies the code
some more.

- Remove Profile::menuIndexAsInt() method, now it's not used anywhere

- This also fixes an issue with sorting the the Default/fallback profile,
since commit ce31d0e2 it would be always sorted at the top of the
list (to signify that it's a special, read-only profile...etc), sorting
by menu index breaks that behaviour:
 - create a new profile by editing the Defualt/fallback profile, and give
   the new profile a name (Defualt is reserved of course)
 - open the profile manager dialog, you'll find the Default/fallback
   profile not at the top of the list, until you restart Konsole

This issue was reported by Kurt in
!351 (comment 197432)

(cherry picked from commit 646408dd)