patch 8.1.0811: too many #ifdefs

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, the final chapter.

src/spell.h

@@ -38,11 +38,7 @@
 
 /* Type used for indexes in the word tree need to be at least 4 bytes.  If int
  * is 8 bytes we could use something smaller, but what? */
-#if VIM_SIZEOF_INT > 3
 typedef int idx_T;
-#else
-typedef long idx_T;
-#endif
 
 typedef int salfirst_T;