Deflake paint test by working around flaky focus behavior

paint-neg-z-order-descendants-into-scrolling-contents-layer.html is a
variant of do-not-paint-outline-into-composited-scrolling-contents.html
which focuses a textarea, but the test does not actually need focus.
This was flaky due to crbug.com/1445159, but the test doesn't need
focus at all, so we can work around this focus flakiness by removing
focus.

Bug: 1444332, 1303091
Change-Id: I25f383c02207f5a5b992d38cd1ca4549c3b14345
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4527064
Reviewed-by: Traian Captan <tcaptan@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1143455}