Fixing an MSVC warning about double "const" data type qualifier

in the code merged from MySQL-5.6:

  const CHARSET_INFO* -> CHARSET_INFO*

(CHARSET_INFO already has the "const" qualifier in MariaDB, inlike in MySQL)

include/m_ctype.h

@@ -749,7 +749,7 @@ size_t my_strxfrm_pad_desc_and_reverse(CHARSET_INFO *cs,
 
 my_bool my_charset_is_ascii_compatible(CHARSET_INFO *cs);
 
-const MY_CONTRACTIONS *my_charset_get_contractions(const CHARSET_INFO *cs,
+const MY_CONTRACTIONS *my_charset_get_contractions(CHARSET_INFO *cs,
                                                    int level);
 
 extern size_t my_vsnprintf_ex(CHARSET_INFO *cs, char *to, size_t n,

strings/ctype-uca.c

@@ -19619,7 +19619,7 @@ my_uca_alloc_contractions(MY_CONTRACTIONS *contractions,
 */
 
 const MY_CONTRACTIONS *
-my_charset_get_contractions(const CHARSET_INFO *cs, int level)
+my_charset_get_contractions(CHARSET_INFO *cs, int level)
 {
   return (cs->uca != NULL) && (cs->uca->level[level].contractions.nitems > 0) ?
           &cs->uca->level[level].contractions : NULL;