Remove unused methods in platform/FileSystem

Will do the same cleanup in chrome side as well.

BUG=none (remotely for 257852)
TEST=compile

Review URL: https://chromiumcodereview.appspot.com/21471003

git-svn-id: svn://svn.chromium.org/blink/trunk@155335 bbb929c8-8fbe-4397-9dbb-9b2b20218538