DELETE FROM ... USING ... statements with the following type of 
ambiguous aliasing gave unexpected results:
DELETE FROM t1 AS alias USING t1, t2 AS alias WHERE t1.a = alias.a;
This query would leave table t1 intact but delete rows from t2.
Fixed by changing DELETE FROM ... USING syntax so that only alias 
references (as opposed to alias declarations) may be used in FROM.


mysql-test/r/delete.result:
  Bug#30234: Test Result
mysql-test/t/delete.test:
  Bug#30234: Test Case
sql/sql_yacc.yy:
  Bug#30234:
  - Added parser rule table_alias_ref_list that contains a list of table 
    aliases only.
  - Added parser rule table_alias_ref that sets the TL_OPTION_ALIAS in 
    order to turn off semantic checking that applies only for table names.