Core: Remove IsHostThread

The core no longer cares which thread is the host thread.

Cleaning up Android's HostThreadLock is left for another PR, in part
because the HostThreadLock in NativeConfig.cpp still serves a purpose,
and in part to make any issues easier to bisect.

Source/UnitTests/UnitTestsMain.cpp

@@ -25,7 +25,6 @@ int main(int argc, char** argv)
 {
   fmt::print(stderr, "Running main() from UnitTestsMain.cpp\n");
   Common::RegisterMsgAlertHandler(TestMsgHandler);
-  Core::DeclareAsHostThread();
 
   ::testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();