Remove invalid `continue` calls from WP_Tax_Query::get_sql_for_clause().

This was leftover code from the previous implementation, which used a `foreach()`
loop. See [29901].

Props nofearinc.
See #29738, #29718.

git-svn-id: https://develop.svn.wordpress.org/trunk@29931 602fd350-edb4-49c9-b593-d223f7449a82