Remove obsolete code allowing 1-indexing the monster list

Having selectable base indexes for the monster list doesn't seem
likely to be needed in the future any more, now that the code for
the monster list is stable. Additionally, the functionality in
question has bitrotted heavily (e.g. many "loops over all
permonsts" start at a hardcoded 0, which wouldn't work with a
1-indexed monster list). As a result, removing the relevant code
in makedefs makes it clearer what can and can't be assumed about
the code, reducing the risk of bugs in the future.

Thanks to FIQ for mentioning that this could be an issue.

util/makedefs.c

@@ -2224,9 +2224,6 @@ do_permonst()
     Fprintf(ofp, "%s", Dont_Edit_Code);
     Fprintf(ofp, "#ifndef PM_H\n#define PM_H\n");
 
-    if (strcmp(mons[0].mname, "playermon") != 0)
-        Fprintf(ofp, "\n#define\tPM_PLAYERMON\t(-1)");
-
     for (i = 0; mons[i].mlet; i++) {
         SpinCursor(3);