rts: Remove incorrect assertions around MSG_THROWTO messages

Previously we would assert that threads which are sending a MSG_THROWTO message must have their blocking status be blocked on the message. In the usual case of a thread throwing to another thread this is guaranteed by stg_killThreadzh. However, throwToSelf, used by the GC to kill threads which ran out of heap, failed to guarantee this.

Noted while debugging #17785 (closed).