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.

include/decl.h

@@ -834,8 +834,7 @@ struct instance_globals {
     struct mkroom rooms[(MAXNROFROOMS + 1) * 2];
     struct mkroom *subrooms;
     dlevel_t level; /* level map */
-    long moves;
-    long monstermoves; /* moves and monstermoves diverge when player is Fast */
+    long moves; /* turn counter */
     long wailmsg;
     struct obj *migrating_objs; /* objects moving to another dungeon level */    
     struct obj *billobjs; /* objects not yet paid for */