Remove Chrome To Mobile user pref cleanup code.

Integrated desktop Chrome To Mobile was removed in r212219.
That same CL added this feature-specific user pref cleanup.
It's safe to remove the cleanup now (two releases later).

BUG=260121
TEST=NONE
R=battre@chromium.org

Review URL: https://codereview.chromium.org/26912004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228234 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228234