[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

include/swift/Threading/Impl/Pthreads.h

@@ -54,7 +54,6 @@ using thread_id = ::pthread_t;
 
 inline thread_id thread_get_current() { return ::pthread_self(); }
 
-thread_id thread_get_main();
 bool thread_is_main();
 
 inline bool threads_same(thread_id a, thread_id b) {