Replace Win32 GC_delete_gc_thread with GC_delete_gc_thread_no_free

Replace Win32 GC_delete_gc_thread with GC_delete_gc_thread_no_free

* win32_threads.c (GC_delete_gc_thread): Rename to
GC_delete_gc_thread_no_free; remove GC_INTERNAL_FREE call; update
comment.
* win32_threads.c (GC_delete_thread, GC_suspend, GC_DllMain): Replace
GC_delete_gc_thread call with GC_delete_gc_thread_no_free one (since
GC_win32_dll_threads is true thus there is no entry to free).
* win32_threads.c (GC_delete_thread): Change type of "t" local
variable from GC_thread to GC_vthread.
* win32_threads.c (GC_stop_world): Update comment.
* win32_threads.c (GC_pthread_join, GC_pthread_detach): Replace
GC_delete_gc_thread call with GC_delete_gc_thread_no_free and
GC_INTERNAL_FREE calls (since GC_win32_dll_threads is false).
* win32_threads.c (GC_pthread_detach): Add assertion that
GC_win32_dll_threads is false.