Revert "xvfb.py: explicitly specify -maxclients=512"

This reverts commit 7b3ea4c4.

Reason for revert: Tests on linux-trusty-rel are failing

Get error message: "Unrecognized option: -maxclients" when starting X server.

Sample failing job: https://ci.chromium.org/p/chromium/builders/ci/linux-trusty-rel/16372

Original change's description:
> xvfb.py: explicitly specify -maxclients=512
>
> When running ash_unittests on CloudTop instance, many tests would fail
> with "Maximum number of clients reached" X11 error. This does not repro
> on physical gLinux workstation though. Adding explicitly -maxclients
> parameter to make it consistent across platforms.
>
> Bug: None
> Test: Run testing/xvfb.py out/Release/ash_unittests
> Change-Id: Iac41378470d87df4b5d8140b3ee03a9a93a8c5ba
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438577
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#812336}

TBR=dpranke@google.com,xiaohuic@chromium.org

Change-Id: I814c0cec7a6b36eaf498838e5ecb27c3564d8ae6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440720Reviewed-by: Jiewei Qian  <qjw@chromium.org>
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812484}

testing/xvfb.py

@@ -164,8 +164,7 @@ def _run_with_xvfb(cmd, env, stdoutfile, use_openbox, use_xcompmgr):
       signal.signal(signal.SIGUSR1, signal.SIG_IGN)
       xvfb_proc = subprocess.Popen(
           ['Xvfb', display, '-screen', '0', '1280x800x24', '-ac',
-           '-maxclients', '512', '-nolisten', 'tcp', '-dpi', '96',
-           '+extension', 'RANDR'],
+           '-nolisten', 'tcp', '-dpi', '96', '+extension', 'RANDR'],
           stderr=subprocess.STDOUT, env=env)
       signal.signal(signal.SIGUSR1, set_xvfb_ready)
       for _ in range(10):