Remove deprecated maniphest.find Conduit method

Summary: This method has been deprecated for a long time (see D2887 and D6336).

Test Plan:

> echo '{}' | arc --conduit-uri=http://phabricator.local call-conduit 'maniphest.find'
Waiting for JSON parameters on stdin...
{"error":"ERR-CONDUIT-CALL","errorMessage":"ERR-CONDUIT-CALL: ","response":null}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11117