commitextras: delete now that it's in the core repo

Authored by martinvonz.

Description

commitextras: delete now that it's in the core repo

It was added to the core repo in 77c0c36654c8 (commitextras: move fb
extension to core which add extras to a commit, 2017-07-12).

Differential Revision: https://phab.mercurial-scm.org/D3566

Details