omit mysql8 dupe key alias for INSERT..FROM SELECT

omit mysql8 dupe key alias for INSERT..FROM SELECT

Fixed issue in MySQL dialect where using INSERT..FROM SELECT in combination
with ON DUPLICATE KEY UPDATE would erroneously render on MySQL 8 and above
the "AS new" clause, leading to syntax failures.  This clause is required
on MySQL 8 to follow the VALUES clause if use of the "new" alias is
present, however is not permitted to follow a FROM SELECT clause.

Fixes: #11731
Change-Id: I254a3db4e9dccd9a76b11fdfe6e38a064ba0b5cf