Remove an obsolete layout test for shadow content pseudo element

LayoutTest/fast/dom/shadow/content-pseudo-element-in-older-shadow.html
passes the layout test currently, but this is based on the old assumption
that style in the sibling shadow root can be applied.
This contradicts what crbug.com/355674 is trying to fix.

I'll add a similar test when I commit a real fix for crbug/355674.

BUG=355674
TEST=should not fail

Review URL: https://codereview.chromium.org/298703006

git-svn-id: svn://svn.chromium.org/blink/trunk@174611 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Also include crash logs for flaky failures

BUG=374167
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/298973003

git-svn-id: svn://svn.chromium.org/blink/trunk@174612 bbb929c8-8fbe-4397-9dbb-9b2b20218538