file_manager: Remove unused code for "format-device"

The code is no longer in use. In fact, "format-device" is not listed in
manifest.json as internal tasks.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/59743007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233592 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233592