Remove unused FileDescriptor::Reset().

The Reset() method was not directly used anywhere.

BUG=b:34284069
TEST=Ran unittests. grep for 'Reset()' in update_engine.

Change-Id: I8db0f17c43a36b03acbc22cdfa35a3e6042c3e75
(cherry picked from commit 4a53c767)
Reviewed-on: https://chromium-review.googlesource.com/641956


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>