Use volumeId instead of mountPath in fileBrowserPrivate.removeMount()

Since mountPath is being deprecated, this patch makes the removeMount() use volumeId. This significantly simplifies the code.

TEST=Tested manually with an archive.
BUG=316077

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=248147

R=benwells@chromium.org, kinaba@chromium.org

Review URL: https://codereview.chromium.org/135183013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248666 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248666