cleanup: remove unused keyinfo flag

HA_UNIQUE_CHECK was
* only used internally by MyISAM/Aria
* only used for internal temporary tables (for DISTINCT)
* never saved in frm
* saved in MYI/MAD but only for temporary tables
* only set, never checked

it's safe to remove it and free the bit (there are only 16 of them)

include/my_base.h

@@ -268,17 +268,15 @@ enum ha_base_keytype {
 
 #define HA_NOSAME		 1U	/* Set if not dupplicated records */
 #define HA_PACK_KEY		 2U	/* Pack string key to previous key */
-#define HA_AUTO_KEY		 16U
+#define HA_AUTO_KEY		 16U    /* MEMORY/MyISAM/Aria internal */
 #define HA_BINARY_PACK_KEY	 32U	/* Packing of all keys to prev key */
 #define HA_FULLTEXT		128U    /* For full-text search */
-#define HA_UNIQUE_CHECK		256U	/* Check the key for uniqueness */
 #define HA_SPATIAL		1024U   /* For spatial search */
 #define HA_NULL_ARE_EQUAL	2048U	/* NULL in key are cmp as equal */
 #define HA_GENERATED_KEY	8192U	/* Automatically generated key */
 
         /* The combination of the above can be used for key type comparison. */
-#define HA_KEYFLAG_MASK (HA_NOSAME | HA_AUTO_KEY | \
-                         HA_FULLTEXT | HA_UNIQUE_CHECK | \
+#define HA_KEYFLAG_MASK (HA_NOSAME | HA_AUTO_KEY | HA_FULLTEXT | \
                          HA_SPATIAL | HA_NULL_ARE_EQUAL | HA_GENERATED_KEY)
 
 /*

storage/maria/ma_create.c

@@ -980,7 +980,6 @@ int maria_create(const char *name, enum data_file_type datafile_type,
   for (i=0; i < uniques ; i++)
   {
     tmp_keydef.keysegs=1;
-    tmp_keydef.flag=		HA_UNIQUE_CHECK;
     tmp_keydef.block_length=	(uint16) maria_block_size;
     tmp_keydef.keylength=	MARIA_UNIQUE_HASH_LENGTH + pointer;
     tmp_keydef.minlength=tmp_keydef.maxlength=tmp_keydef.keylength;

storage/myisam/mi_create.c

@@ -754,7 +754,6 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
   for (i=0; i < uniques ; i++)
   {
     tmp_keydef.keysegs=1;
-    tmp_keydef.flag=		HA_UNIQUE_CHECK;
     tmp_keydef.block_length=	(uint16)myisam_block_size;
     tmp_keydef.keylength=	MI_UNIQUE_HASH_LENGTH + pointer;
     tmp_keydef.minlength=tmp_keydef.maxlength=tmp_keydef.keylength;