MEDIUM: mux-fcgi: do not try to stop sending streams on blocked mux

This is essentially the same change as applied to mux-h2 in previous commit
"MEDIUM: mux-h2: do not try to stop sending streams on blocked mux". The
goal is to make sure we don't need to keep the item in the send_wait list
until it's executed so that we can later merge it with the recv_wait list.
No performance changes were observed.
1 file changed