Remove g.monstermoves

It's redundant with g.moves, so there is no more need for it.

Way, way back, it looks like g.moves and g.monstermoves can and did
desync, where g.moves would track the amount of moves the player had
gotten (and would therefore increase faster if the player were hasted)
and g.monstermoves would track the amount of monster move cycles, aka
turns. But this has not been the case for a long time, and they both
increment together in the same location in allmain.c. There are no
longer any cases where they will not be the same value.

This is a save-breaking change because it changes struct
instance_globals, but I have not updated the editlevel in this commit.

src/mhitm.c

@@ -341,7 +341,7 @@ mattackm(register struct monst *magr, register struct monst *mdef)
      * some cases, in which case this still counts as its move for the round
      * and it shouldn't move again.
      */
-    magr->mlstmv = g.monstermoves;
+    magr->mlstmv = g.moves;
 
     /* controls whether a mind flayer uses all of its tentacle-for-DRIN
        attacks; when fighting a headless monster, stop after the first