Only define GTEST_IS_THREADSAFE to 1, not 0

According to the comments, "Feature-indicating macros",
such as GTEST_IS_THREADSAFE should be defined to 1 when supported
and otherwise undefined (never 0).

PiperOrigin-RevId: 513944266
Change-Id: I0f5c8bed107a5f20e957ec7c70339540ca2fe831

googletest/include/gtest/internal/gtest-port.h

@@ -790,8 +790,6 @@ typedef struct _RTL_CRITICAL_SECTION GTEST_CRITICAL_SECTION;
       !defined(GTEST_OS_WINDOWS_RT)) ||                                \
      GTEST_HAS_PTHREAD)
 #define GTEST_IS_THREADSAFE 1
-#else
-#define GTEST_IS_THREADSAFE 0
 #endif
 
 #endif  // GTEST_IS_THREADSAFE