cleanup: simplify opening of priv tables

prepare TABLE_LIST in a loop and just before opening
don't store TABLE_LIST inside Grant_table_base.

sql/table.cc

@@ -70,8 +70,6 @@ LEX_CSTRING GENERAL_LOG_NAME= {STRING_WITH_LEN("general_log")};
 LEX_CSTRING SLOW_LOG_NAME= {STRING_WITH_LEN("slow_log")};
 
 LEX_CSTRING TRANSACTION_REG_NAME= {STRING_WITH_LEN("transaction_registry")};
-LEX_CSTRING MYSQL_USER_NAME= {STRING_WITH_LEN("user")};
-LEX_CSTRING MYSQL_DB_NAME= {STRING_WITH_LEN("db")};
 LEX_CSTRING MYSQL_PROC_NAME= {STRING_WITH_LEN("proc")};
 
 /* 

sql/table.h

@@ -2952,7 +2952,7 @@ extern LEX_CSTRING INFORMATION_SCHEMA_NAME;
 extern LEX_CSTRING MYSQL_SCHEMA_NAME;
 
 /* table names */
-extern LEX_CSTRING MYSQL_USER_NAME, MYSQL_DB_NAME, MYSQL_PROC_NAME;
+extern LEX_CSTRING MYSQL_PROC_NAME;
 
 inline bool is_infoschema_db(const LEX_CSTRING *name)
 {