Remove obsolete sandbox disabling logic from runtest.py.

Remove obsolete sandbox disabling logic from runtest.py.

Previously, we disabled the SUID sandbox by setting CHROME_DEVEL_SANDBOX="" in
the following cases:
- If LSan or TSan were enabled. However, we already pass --no-sandbox in this
case, so setting the env variable to an empty string is unnecessary.
- If the SUID sandbox binary was not present or not configured correctly.
This was useful when SUID sandbox was being rolled out. However, according to
https://code.google.com/p/chromium/wiki/LinuxSUIDSandboxDevelopment
the sandbox binary must now be present on all bots. If it isn't, it makes more
sense to fail than to disable the sandbox. Also, according to the same doc,
setting the env variable to "" is no longer a supported way of disabling the
sandbox.

BUG=245376
R=stip@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=293832

[earthdok, 2015-01-27 19:45:55]

Mike, please take a look. +jln@ FYI

The same change will need to be done to src/testing/test_env.py

[jln (very slow on Chromium), 2015-01-27 20:08:38]

Thanks a lot for doing that! lgtm on my side

You may want to link to crbug.com/245376

[ghost stip (do not use), 2015-01-27 21:49:52]

owner lgtm

[earthdok, 2015-01-27 23:01:08]

The CQ bit was checked by earthdok@chromium.org

[commit-bot: I haz the power, 2015-01-27 23:01:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/881743004/20001

[commit-bot: I haz the power, 2015-01-27 23:04:41]

Committed patchset #2 (id:20001) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=293832