NaCl cleanup: Remove assignment to g_is_main_thread

NaCl cleanup: Remove assignment to g_is_main_thread

The variable g_is_main_thread is only read from dead code in
irt_manifest.c in the NaCl repo.  It is read from the old SRPC-based
implementation of open_resource(), which isn't accessible because it's
overridden by the Chromium side's implementation of open_resource().

Removing this assignment will allow us to remove irt_manifest.c, which
defines g_is_main_thread.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3864
TEST=trybots

Committed: https://crrev.com/848bcea8ce9db5ed0c070828133a4cd66934c272
Cr-Commit-Position: refs/heads/master@{#308172}

[Mark Seaborn, 2014-12-12 18:03:15]

mseaborn@chromium.org changed reviewers:
+ jvoung@chromium.org, teravest@chromium.org

[teravest, 2014-12-12 18:10:52]

lgtm

[Mark Seaborn, 2014-12-12 19:31:53]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2014-12-12 19:32:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/799023003/1

[commit-bot: I haz the power, 2014-12-12 21:59:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-12 22:01:05]

Patchset 1 (id:??) landed as
https://crrev.com/848bcea8ce9db5ed0c070828133a4cd66934c272
Cr-Commit-Position: refs/heads/master@{#308172}