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.
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git-svn-id: http://core.svn.wordpress.org/trunk@29683 1a063a9b-81f0-0310-95a4-ce76da25c4cd