BUG/MINOR: epoll/threads: only call epoll_ctl(DEL) on polled FDs

BUG/MINOR: epoll/threads: only call epoll_ctl(DEL) on polled FDs

Commit d9e7e36 ("BUG/MEDIUM: epoll/threads: use one epoll_fd per thread")
addressed an issue with the polling and required that cloned FDs are removed
from all polling threads on close. But in fact it does it for all bound
threads, some of which may not necessarily poll the FD. This is harmless,
but it may also make it harder later to deal with FD migration between
threads. Better use polled_mask which only reports threads still aware
of the FD instead of thread_mask.

This fix should be backported to 1.8.