extmsg: fix Message-ID search for non-xap_helper users

extmsg: fix Message-ID search for non-xap_helper users

commit c3d0295bd was intended to avoid head-of-line blocking for
--xapian-helpers (-X) deployments.  Unfortunately, that broke
searches for deployments not using --xapian-helpers by causing
the ExtMsg to get queued redundantly into the event loop.  So
perform the `requeue' procedure exactly once and simplify
ExtMsg->event_step by eliminating the $sync parameter.

Fixes: c3d0295bd (www: extmsg: async partial Message-ID search, 2025-05-12)
Reported-by: Kyle Meyer <kyle@kyleam.com>
Link: https://public-inbox.org/meta/877bwva3s4.fsf@kyleam.com/