Revert "Keep the DTLS timer running after the end of the handshake if appropriate"

This commit erroneously kept the DTLS timer running after the end of the
handshake. This is not correct behaviour and shold be reverted.

This reverts commit f7506416.

Fixes #7998

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8047)