mash cleanup: Remove unused window title shown property

I performed a quick codesearch audit from the PropertyMirror removal CL:
http://crrev.com/c/1603215/3/ash/public/cpp/mus_property_mirror_ash.cc#b75
kTitleShownKey is the only obviously unused property.

Bug: 961478
Test: No behavior change
Change-Id: I43fcd9d5968c4fba4aa8a4a2af45b5f9f9372366
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666559
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670358}
2 files changed