Cleanup: Remove unused DB_TABLE_IS_BEING_USED

The error code DB_TABLE_IS_BEING_USED was orphaned in
commit ddc663ef (MySQL 3.23.37).
Let us finally remove it.

mysql-test/suite/innodb/r/innodb-wl5522-debug.result

@@ -453,7 +453,7 @@ restore: t1 .ibd and .cfg files
 SET SESSION debug_dbug=@saved_debug_dbug;
 SET SESSION debug_dbug="+d,ib_import_open_tablespace_failure";
 ALTER TABLE t1 IMPORT TABLESPACE;
-ERROR HY000: Got error 44 'Tablespace not found' from ./test/t1.ibd
+ERROR HY000: Got error 43 'Tablespace not found' from ./test/t1.ibd
 SET SESSION debug_dbug=@saved_debug_dbug;
 restore: t1 .ibd and .cfg files
 SET SESSION debug_dbug="+d,ib_import_check_bitmap_failure";
@@ -922,7 +922,7 @@ ERROR HY000: Tablespace has been discarded for table `t1`
 restore: t1 .ibd and .cfg files
 SET SESSION debug_dbug="+d,dict_tf_to_fsp_flags_failure";
 ALTER TABLE t1 IMPORT TABLESPACE;
-ERROR HY000: Got error 39 'Data structure corruption' from ./test/t1.ibd
+ERROR HY000: Got error 38 'Data structure corruption' from ./test/t1.ibd
 SET SESSION debug_dbug=@saved_debug_dbug;
 DROP TABLE t1;
 unlink: t1.ibd

mysql-test/suite/innodb_zip/r/wl5522_debug_zip.result

@@ -88,7 +88,7 @@ restore: t1 .ibd and .cfg files
 SET SESSION debug_dbug=@saved_debug_dbug;
 SET SESSION debug_dbug="+d,ib_import_open_tablespace_failure";
 ALTER TABLE t1 IMPORT TABLESPACE;
-ERROR HY000: Got error 44 'Tablespace not found' from ./test/t1.ibd
+ERROR HY000: Got error 43 'Tablespace not found' from ./test/t1.ibd
 SET SESSION debug_dbug=@saved_debug_dbug;
 restore: t1 .ibd and .cfg files
 SET SESSION debug_dbug="+d,ib_import_check_bitmap_failure";
@@ -388,7 +388,7 @@ ERROR HY000: Tablespace has been discarded for table `t1`
 restore: t1 .ibd and .cfg files
 SET SESSION debug_dbug="+d,dict_tf_to_fsp_flags_failure";
 ALTER TABLE t1 IMPORT TABLESPACE;
-ERROR HY000: Got error 39 'Data structure corruption' from ./test/t1.ibd
+ERROR HY000: Got error 38 'Data structure corruption' from ./test/t1.ibd
 SET SESSION debug_dbug=@saved_debug_dbug;
 DROP TABLE t1;
 unlink: t1.ibd

storage/innobase/handler/ha_innodb.cc

@@ -1758,9 +1758,6 @@ convert_error_code_to_mysql(
 	case DB_TABLE_IN_FK_CHECK:
 		return(HA_ERR_TABLE_IN_FK_CHECK);
 
-	case DB_TABLE_IS_BEING_USED:
-		return(HA_ERR_WRONG_COMMAND);
-
 	case DB_TABLE_NOT_FOUND:
 		return(HA_ERR_NO_SUCH_TABLE);
 

storage/innobase/include/db0err.h

@@ -51,7 +51,6 @@ enum dberr_t {
 	DB_TABLE_NOT_FOUND,
 	DB_MUST_GET_MORE_FILE_SPACE,	/*!< the database has to be stopped
 					and restarted with more file space */
-	DB_TABLE_IS_BEING_USED,
 	DB_TOO_BIG_RECORD,		/*!< a record in an index would not fit
 					on a compressed page, or it would
 					become bigger than 1/2 free space in

storage/innobase/ut/ut0ut.cc

@@ -366,8 +366,6 @@ ut_strerr(
 		return("Table not found");
 	case DB_MUST_GET_MORE_FILE_SPACE:
 		return("More file space needed");
-	case DB_TABLE_IS_BEING_USED:
-		return("Table is being used");
 	case DB_TOO_BIG_RECORD:
 		return("Record too big");
 	case DB_TOO_BIG_INDEX_COL: