R1:422b2f2b891e

Remove deprecated `maniphest.find` Conduit method

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

Test Plan:
```lang=bash
> 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…