EmptyTrashIOTask removes the whole '.Trash' dir

... instead of removing the two subdirs 'files' and 'info' of each
'.Trash' dir.

This simplifies the logic of EmptyTrashIOTask.

BUG=None
TEST=unit_tests --gtest_filter=EmptyTrashIOTaskTest.*

Change-Id: I1970589cdf50ae425f7bde98d42783c58d054560
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5606103


Commit-Queue: François Degros <fdegros@chromium.org>
Reviewed-by: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313190}