[Threading] Remove thread_get_main().

Removing thread_get_main() means we don't need a static initializer on
Darwin, which means we can delete Darwin.cpp as well.  We can also delete
Nothreads.cpp while we're about it, because there's nothing in that file.

rdar://90776105

lib/Threading/Linux.cpp

@@ -42,14 +42,9 @@ MainThreadRememberer rememberer;
 using namespace swift;
 using namespace threading_impl;
 
-thread_id
-swift::threading_impl::thread_get_main() {
-  return rememberer.main_thread();
-}
-
 bool
 swift::threading_impl::thread_is_main() {
-  return pthread_equal(pthread_self(), thread_get_main());
+  return pthread_equal(pthread_self(), rememberer.main_thread());
 }
 
 void