Prune mcrypt extension

Summary:
Don't ask to build mcrypt extension.

Don't install dependant lib.

Reference: https://wiki.php.net/rfc/mcrypt-viking-funeral

Fixes T1436.

Test Plan: Build

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1436

Differential Revision: https://devcentral.nasqueron.org/D2336