sql_yacc.yy: get rid of the rules "opt_if_not_exists_table_element" and "opt_if_exists_table_element"

This is a clean-up task that can be easily done after MDEV-8093.

There are two pairs of very similar rules:

• opt_if_exists vs opt_if_exists_table_element
• opt_if_not_exists vs opt_if_not_exists_table_element

which all handle IF EXISTS and IF NOT EXISTS clauses.

The "opt_if_xxx_table_element" rules use Lex->check_exists as a storage and are used in column and key context of ALTER TABLE:

ALTER TABLE t1 DROP INDEX IF EXISTS f1;

The "opt_if_xxx" rules are based on DDL_options_st and used in a schema object context:

CREATE TABLE IF NOT EXISTS t1 (a int);

DROP FUNCTION IF EXISTS f1;

We'll modify sql_yacc.yy to use the DDL_options_st based rules in all contexts and will get rid of:

• The "opt_if_exists_table_element" rule
• The "opt_if_not_exists_table_element" rule
• The "check_exists" member in LEX.