cryptohome: Remove unnecessary include of mock_cryptohome_library.h

BUG=298605
TEST=none
R=hashimoto@chromium.org

Review URL: https://codereview.chromium.org/24618004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225341 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225341