Doc: in v11 release notes, remove item about citext_pattern_ops.

Doc: in v11 release notes, remove item about citext_pattern_ops.

The entry for commit f24649976 claimed that citext_pattern_ops could be
used for LIKE index searches, but in fact it cannot.  That patch only
created the opclass and some related operators, without doing anything
to teach the planner about how to use it.  The opclass is basically
useless in this unfinished state, which is why nothing was added to the
main documentation about it, and really we shouldn't have said anything
in the release notes either.  So remove the entry.

Discussion: https://postgr.es/m/CAKqncch4eBt2c8ddNVxzcVh3fFhdk54QoDMzpKgpqor4gA-wcQ@mail.gmail.com