Files.app: Remove the unused unmount icon.

The unmount icon of the directory tree is moved to the navigation list.
This Cl removs the unused unmount icon in the direcotry tree.

BUG=324419
TEST=manually, mount zip file and unmount it.

Review URL: https://codereview.chromium.org/96503003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238023 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238023