Remove contact search doc

The search mechanism documented here currently doesn't work:

user@apertis:~$ folks-inspect search foo

(folks-inspect:13376): folks-WARNING **: 01:48:07.385: backend-store.vala:434: Error preparing Backend 'telepathy': The name org.freedesktop.Telepathy.AccountManager was not provided by any .service files

This functionality is currently not a priority, drop the documentation
since it doesn't work.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>