[NaCl SDK] Remove gtest_main.cc from libgtest

This file is not supposed to be part of libgtest but in its
own library called libgtest_main.a.  Its also fairly
useless right now in the context of PPAPI (all our gtest
examples currently use ppapi_simple and define their own
entry point).  In the long run we could re-enable this
if it becomes useful, but really we should probably just
remove the gtest libraries from the SDK completely since
shipping gtest in library form is not recommended practice.

Review URL: https://codereview.chromium.org/958233004

Cr-Commit-Position: refs/heads/master@{#318932}

native_client_sdk/src/libraries/gtest/library.dsc

@@ -13,7 +13,6 @@
         'gtest.cc',
         'gtest-death-test.cc',
         'gtest-filepath.cc',
-        'gtest_main.cc',
         'gtest-port.cc',
         'gtest-printers.cc',
         'gtest-test-part.cc',