Further improvements to ASYNC_WAIT_CTX_clear_fd

Remove call to cleanup function
Use only one loop to find previous element

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 219aa86c)