CLEANUP: backend: use a single variable for removed in srv_cleanup_idle_conns()

CLEANUP: backend: use a single variable for removed in srv_cleanup_idle_conns()

Probably due to older code, there's a boolean variable used to set
another one which is then checked. Also the first check is made under
the lock, which is unnecessary. Let's simplify this and use a single
variable. This only makes the code clearer, it doesn't change the output
code.