misc: move mp_cancel from stream.c to thread_tools.c

It seems a bit inappropriate to have dumped this into stream.c, even if
it's roughly speaking its main user. At least it made its way somewhat
unfortunately to other components not related to the stream or demuxer
layer at all.

I'm too greedy to give this weird helper its own file, so dump it into
thread_tools.c.

Probably a somewhat pointless change.