MINOR: cli/server: don't take thread isolation to check for srv-removable

MINOR: cli/server: don't take thread isolation to check for srv-removable

Thanks to the previous commits, we now know that "wait srv-removable"
does not require thread isolation, as long as 3372a2ea00 ("BUG/MEDIUM:
queues: Stricly respect maxconn for outgoing connections") and c880c32b16
("MINOR: stream: decrement srv->served after detaching from the list")
are present. Let's just get rid of thread_isolate() here, which can
consume a lot of CPU on highly threaded machines when removing many
servers at once.