core: avoid double loop in ap_filter_output_pending().

Since ap_filter_output_pending() is looping on the relevant filters already,
we don't need to use ap_filter_should_yield() to check upstream filters.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836240 13f79535-47bb-0310-9956-ffa450edef68