test.cmake: remove unnecessary gtest include path

only googletest/include is needed for the gtest.h sub-includes.

Change-Id: Ib821a596103377c2f7a0df5f99ead209874c8ee6
diff --git a/test/test.cmake b/test/test.cmake
index f584433..9f9e89e 100644
--- a/test/test.cmake
+++ b/test/test.cmake
@@ -349,7 +349,6 @@
   endif ()
 
   include_directories(
-    "${AOM_ROOT}/third_party/googletest/src/googletest/src"
     "${AOM_ROOT}/third_party/googletest/src/googletest/include")
 
   if (AOM_DISABLE_GTEST_CMAKE)