Revert "Abort on unrecognised warning alerts"

This reverts commit 77a6be4d

.

There were some unexpected side effects to this commit, e.g. in SSLv3 a
warning alert gets sent "no_certificate" if a client does not send a
Certificate during Client Auth. With the above commit this causes the
connection to abort, which is incorrect. There may be some other edge cases
like this so we need to have a rethink on this.

Reviewed-by: Tim Hudson <tjh@openssl.org>