Re-enable MSVC++ C4389 warning in CmdHelperEq()

C4389 was inhibited in commit 4b83461 making behavior inconsistent with
other compilers.

googletest/include/gtest/gtest.h

@@ -1386,11 +1386,9 @@ AssertionResult CmpHelperEQ(const char* lhs_expression,
                             const char* rhs_expression,
                             const T1& lhs,
                             const T2& rhs) {
-GTEST_DISABLE_MSC_WARNINGS_PUSH_(4389 /* signed/unsigned mismatch */)
   if (lhs == rhs) {
     return AssertionSuccess();
   }
-GTEST_DISABLE_MSC_WARNINGS_POP_()
 
   return CmpHelperEQFailure(lhs_expression, rhs_expression, lhs, rhs);
 }