filelog: stop proxying "opener" (API)
The last consumer of it in upgrade code was removed as part of the
previous commit. This attribute is revlog specific (because it
assumes the existence of a vfs for performing I/O on tracked file
data) and therefore isn't appropriate for a generic storage interface.
So nuke it.
Differential Revision: https://phab.mercurial-scm.org/D4749
(binary:application/octet-stream)