linux: fix abort() on epoll_ctl() race condition

Don't check the return value of epoll_ctl(EPOLL_CTL_DEL). When the
file descriptor is closed, we're potentially racing with another
thread and that means the errno is not a reliable indicator of
the actual error.

The other event mechanisms (kqueue, event ports) are not affected
because:

* kqueue returns either EBADF or ENOENT. Both are handled by libuv.
* event ports rearms all file descriptors on each call to port_getn().

Fixes joyent/node#4558.