Remove dead Android code from content test launcher.

Android browser tests do not provide a ContentMainDelegate through
the TestLauncherDelegate, they set it via content_main.h's
SetContentMainDelegate() method. They do have have the test launcher
run ContentMain(), nor does BrowserTestBase run ContentMain(). Instead
Android browser tests mimic the behaviour of ContentMain and call
BrowserMain() directly in the test process. They ContentMainDelegate
is not used in content test launcher, nor does the ContentMainParams
exist or get used. And Android browser tests run child processes as
threads, so child processes do not need to run ContentMain() from the
content test launcher.

Bug: 961849
Change-Id: Icef9590a1f97b7948229934f66f9f91cf304aeab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606402
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658753}