Enable -Wconversion: mark.c.

Refactoring summary:
- MB_STRNICMP: Inlined.
- MB_STRNCMP: Inlined.

src/nvim/vim.h

@@ -290,17 +290,6 @@ enum {
 # endif
 #endif
 
-/* We need to call mb_stricmp() even when we aren't dealing with a multi-byte
- * encoding because mb_stricmp() takes care of all ascii and non-ascii
- * encodings, including characters with umlauts in latin1, etc., while
- * STRICMP() only handles the system locale version, which often does not
- * handle non-ascii properly. */
-
-# define MB_STRICMP(d, s)       mb_strnicmp((char_u *)(d), (char_u *)(s), \
-    (int)MAXCOL)
-# define MB_STRNICMP(d, s, n)   mb_strnicmp((char_u *)(d), (char_u *)(s), \
-    (int)(n))
-
 #define STRCAT(d, s)        strcat((char *)(d), (char *)(s))
 #define STRNCAT(d, s, n)    strncat((char *)(d), (char *)(s), (size_t)(n))