drive: Remove IsUnrecoverableError()

This function was introduced in r218440 to avoid discarding DB when it doesn't help.
We no longer need this because after r231743, unnecessary deletion of DB is avoided by preserving old DB.

BUG=None
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/53783004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232088 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232088