Revert "Add BUILD.gn for ui/base/user_activity"

This reverts commit a230653deca20e6bebdc37a24ba5447f9cf0c4b6.

Reason for revert: Breaks linux-chromeos-dbg crbug.com/928124

Original change's description:
> Add BUILD.gn for ui/base/user_activity
> 
> ui/base/user_activity causes ui/base to have a dependency on ui/events.
> To remove another instance of this dependency, put user_activity into
> its own component.
> 
> 
> Bug: 921723
> Change-Id: I8c82bcb82b876386c63fa263937280c48ceb43fb
> Reviewed-on: https://chromium-review.googlesource.com/c/1448735
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Daniel Libby <dlibby@microsoft.com>
> Cr-Commit-Position: refs/heads/master@{#628473}

TBR=sadrul@chromium.org,sky@chromium.org,dlibby@microsoft.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 921723, 928124
Change-Id: I63fb4320cba8313b1ffa021b90ddbe2188f1f3e4
Reviewed-on: https://chromium-review.googlesource.com/c/1451477
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628641}
12 files changed