infinitepush: remove backupcommands.py
This file contains command pushbackup and pullbackup which are used to store
things to infinitepush.
This is an advanced functionality which we don't require as of now. Also this
uses shareutil.py from fb-hgext/ which is not there in core. Therefore this
patch deletes the file and the config option which belongs to the backup thing.
If we need this functionality in future, we can always move this file back.
Differential Revision: https://phab.mercurial-scm.org/D2097