Remove the coupling of Client Certificates and Auth Type External for SMTP

Currently client certificates can only be used in combination with the EXTERNAL authentication mechanism. This pull request removes this coupling as there is no necessity for it. The EXTERNAL authentication mechanism could even be used without STARTTLS or SSL/TLS (see e.g. https://tools.ietf.org/html/rfc4422#appendix-A). However, doing so would be a security problem comparable to a cleartext username+password transmission.

With this pull request client certificates can be used in combination with every authentication mechanism present in k9. They can also be used without an authentication mechanism.
Also it is possible to use the EXTERNAL authentication mechanism with an empty string now.

There are also other people who see the need for this: https://groups.google.com/forum/#!topic/k-9-mail/FuUP7oSwYe0