Remove support for SSL3_FLAGS_DELAY_CLIENT_FINISHED

Remove support for SSL3_FLAGS_DELAY_CLIENT_FINISHED

This flag was not set anywhere within the codebase (only read). It could
only be set by an app reaching directly into s->s3->flags and setting it
directly. However that method became impossible when libssl was opaquified.

Even in 1.0.2/1.0.1 if an app set the flag directly it is only relevant to
ssl3_connect(), which calls SSL_clear() during initialisation that clears
any flag settings. Therefore it could take effect if the app set the flag
after the handshake has started but before it completed. It seems quite
unlikely that any apps really do this (especially as it is completely
undocumented).

The purpose of the flag is suppress flushing of the write bio on the client
side at the end of the handshake after the client has written the Finished
message whilst resuming a session. This enables the client to send
application data as part of the same flight as the Finished message.

This flag also controls the setting of a second flag SSL3_FLAGS_POP_BUFFER.
There is an interesting comment in the code about this second flag in the
implementation of ssl3_write:

	/* This is an experimental flag that sends the
	 * last handshake message in the same packet as the first
	 * use data - used to see if it helps the TCP protocol during
	 * session-id reuse */

It seems the experiment did not work because as far as I can tell nothing
is using this code. The above comment has been in the code since SSLeay.

This commit removes support for SSL3_FLAGS_DELAY_CLIENT_FINISHED, as well
as the associated SSL3_FLAGS_POP_BUFFER.

Reviewed-by: Rich Salz <rsalz@openssl.org>