Cleanup path utils in Files app.

This patch cleans up path_util.js, by removing unused methods and simplifying others.

TEST=Tested manually. Partly browser tests.
BUG=320967

Review URL: https://codereview.chromium.org/131743019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245900 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245900