Macro cleanup: Miscellaneous

These were found with -Wunused-macros. There are many more macros which
triggered that warning, but they were primarily part of larger sets of
macros so leave them alone.

src/nvim/mbyte.c

@@ -106,8 +106,6 @@
 #include "nvim/os/os.h"
 #include "nvim/arabic.h"
 
-#define WINBYTE BYTE
-
 typedef struct {
   int rangeStart;
   int rangeEnd;
@@ -365,10 +363,6 @@ enc_alias_table[] =
   {NULL,              0}
 };
 
-#ifndef CP_UTF8
-# define CP_UTF8 65001  /* magic number from winnls.h */
-#endif
-
 /*
  * Find encoding "name" in the list of canonical encoding names.
  * Returns -1 if not found.