fs_lease: remove dangerous method IsValid()
fs_lease: remove dangerous method IsValid()

If this ever returns false, then this is a use-after-free.  We should
stop doing that, even if we have a pool reference.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy