This reverts commit c74347a508.
I'm afraid that change was just all wrong.
* The gitattributes setting is `annex.backend`. Not `annex.backends`.
* `gitattributes` files need to start with a glob, which this commit removed.
* git config's `annex.backends` is NOT the same as the gitattributes
setting, and is not a knob that we should be encouraging users tweak.