MINOR: fd/thread: get rid of thread_mask()

MINOR: fd/thread: get rid of thread_mask()

Since commit d2494e048 ("BUG/MEDIUM: peers/config: properly set the
thread mask") there must not remain any single case of a receiver that
is bound nowhere, so there's no need anymore for thread_mask().

We're adding a test in fd_insert() to make sure this doesn't happen by
accident though, but the function was removed and its rare uses were
replaced with the original value of the bind_thread msak.