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.

src/nvim/message.c

@@ -10,8 +10,6 @@
  * message.c: functions for displaying messages on the command line
  */
 
-#define MESSAGE_FILE            /* don't include prototype for smsg() */
-
 #include <assert.h>
 #include <errno.h>
 #include <inttypes.h>

src/nvim/syntax.c

@@ -223,8 +223,6 @@ struct name_list {
 #define ATTR_OFF (HL_ALL + 1)
 
 
-#define SYN_NAMELEN     50              /* maximum length of a syntax name */
-
 static char *(spo_name_tab[SPO_COUNT]) =
 {"ms=", "me=", "hs=", "he=", "rs=", "re=", "lc="};
 
@@ -6070,8 +6068,6 @@ do_highlight (
   int error = FALSE;
   int color;
   int is_normal_group = FALSE;                  /* "Normal" group */
-# define is_menu_group 0
-# define is_tooltip_group 0
 
   /*
    * If no argument, list current highlighting.