fix build warning

fix build warning

A recent change made plumbing the backend through fsck unncessary.

Left fsck checking backend and skipping operating on key when it could
not find one. Not checking the backend would be a behavior change.
For example the command git-annex fsck --key FOO--bar does nothing
since FOO is not a known backend. If this were removed it would
instead go on and fsck it and warn that no copies exist of the key.
That behavior change seems like it would be fine, but I also have no
reason to make it.