stop removing session cache from memcached

[i110]

as discussed at #1087 (comment), we shouldn't remove tls session cache actively.

Regarding the manual, there are 3 triggers the remove callback gets called:

The remove_session_cb() is called, whenever the SSL engine removes a session from the internal cache. This happens

• when the session is removed because it is expired
• or when a connection was not shutdown cleanly.
• It also happens for all sessions in the internal session cache when SSL_CTX_free is called.

For the first one, currently we're using memcached (and redis in near future) which have their own expiration feature, so there's no need to remove explicitly.
For the second, we would not want to remove a cache entry when a connection is abruptly shut down, which is often the case with the browsers.
For the last, the session cache should also still be alive.

For above reasons, I believe that we should do nothing when the remove events get fired

[kazuho]

Thank you for the PR.

How about removing on_async_resumption_remove as a whole (at the same time stop calling SSL_CTX_sess_set_remove_cb)?

I don't see any need to retain the code.

[i110]

@kazuho Thank you for your review. I removed that.

[kazuho]

Thank you very much! Less code = win.