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

Review URL: https://codereview.chromium.org/799023003

Cr-Commit-Position: refs/heads/master@{#308172}

ppapi/nacl_irt/DEPS

@@ -9,7 +9,6 @@ include_rules = [
   "+native_client/src/public",
   "+native_client/src/shared/srpc/nacl_srpc.h",
   "+native_client/src/untrusted/irt/irt.h",
-  "+native_client/src/untrusted/irt/irt_private.h",
   # The IRT also needs to know the errno and sysconf enums.
   "+native_client/src/trusted/service_runtime/include/sys/errno.h",
   "+native_client/src/trusted/service_runtime/include/sys/unistd.h",

ppapi/nacl_irt/irt_ppapi.cc

@@ -8,7 +8,6 @@
 #include "native_client/src/public/irt_core.h"
 #include "native_client/src/trusted/service_runtime/include/sys/unistd.h"
 #include "native_client/src/untrusted/irt/irt.h"
-#include "native_client/src/untrusted/irt/irt_private.h"
 #include "ppapi/nacl_irt/irt_manifest.h"
 #include "ppapi/nacl_irt/irt_ppapi.h"
 #include "ppapi/nacl_irt/plugin_main.h"
@@ -18,14 +17,6 @@
 static struct PP_StartFunctions g_pp_functions;
 
 int irt_ppapi_start(const struct PP_StartFunctions* funcs) {
-#if !defined(OS_NACL_NONSFI)
-  // Disable NaCl's open_resource() interface on this thread.
-  // TODO(hidehiko): This flag is no longer used because, even in SFI mode,
-  // we switched from SRPC to Chrome IPC for open_resource() implementation.
-  // Remove this flag.
-  g_is_main_thread = 1;
-#endif
-
   g_pp_functions = *funcs;
   return PpapiPluginMain();
 }