MDEV-26372 enforce-storage-engine=InnoDB has no usability as an option to mysqld-install-db

Creation of tables by the three names mysql.user, mysql.host, mysql.db
was being blocked in the function row_mysql_is_system_table().

Since commit 4abb8216 (MDEV-17658),
mysql.user is a view, not a table. Since commit
ead9a34a (MDEV-15851), mysql.host
is not being created at all.

Let us remove the special handling of table names in InnoDB,
and allow mysql.db to be created in InnoDB. The special handling
was originally added in commit e84ef2b7
without any explanation.

Reviewed by: Sergei Golubchik

storage/innobase/row/row0mysql.cc

@@ -100,25 +100,6 @@ static ib_mutex_t row_drop_list_mutex;
 /** Flag: has row_mysql_drop_list been initialized? */
 static bool row_mysql_drop_list_inited;
 
-/*******************************************************************//**
-Determine if the given name is a name reserved for MySQL system tables.
-@return TRUE if name is a MySQL system table name */
-static
-ibool
-row_mysql_is_system_table(
-/*======================*/
-	const char*	name)
-{
-	if (strncmp(name, "mysql/", 6) != 0) {
-
-		return(FALSE);
-	}
-
-	return(0 == strcmp(name + 6, "host")
-	       || 0 == strcmp(name + 6, "user")
-	       || 0 == strcmp(name + 6, "db"));
-}
-
 #ifdef UNIV_DEBUG
 /** Wait for the background drop list to become empty. */
 void
@@ -2374,25 +2355,12 @@ row_create_table_for_mysql(
 
 	DBUG_EXECUTE_IF(
 		"ib_create_table_fail_at_start_of_row_create_table_for_mysql",
-		goto err_exit;
-	);
-
-	trx->op_info = "creating table";
-
-	if (row_mysql_is_system_table(table->name.m_name)) {
-
-		ib::error() << "Trying to create a MySQL system table "
-			<< table->name << " of type InnoDB. MySQL system"
-			" tables must be of the MyISAM type!";
-#ifndef DBUG_OFF
-err_exit:
-#endif /* !DBUG_OFF */
 		dict_mem_table_free(table);
-
 		trx->op_info = "";
+		return DB_ERROR;
+	);
 
-		return(DB_ERROR);
-	}
+	trx->op_info = "creating table";
 
 	trx_start_if_not_started_xa(trx, true);
 
@@ -4196,14 +4164,6 @@ row_rename_table_for_mysql(
 
 	if (high_level_read_only) {
 		return(DB_READ_ONLY);
-
-	} else if (row_mysql_is_system_table(new_name)) {
-
-		ib::error() << "Trying to create a MySQL system table "
-			<< new_name << " of type InnoDB. MySQL system tables"
-			" must be of the MyISAM type!";
-
-		goto funct_exit;
 	}
 
 	trx->op_info = "renaming table";