Remove fd from the list when the engine clears the wait context before pause

This fixes the num of fds added/removed returned by ASYNC_WAIT_CTX_get_changed_fds

Previously, the numbers were not consistent with the fds actually written in
the buffers since the fds that have been both added and removed are explicitly
ignored in the loop.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2581)
(cherry picked from commit f89dd673)