Remove PKA support.

[lambdafu]

PKA is a proposal for automatic public key retrieval over DNS by Werner Koch. These seem to be the authoritative documents:

• http://g10code.com/docs/pka-intro.de.pdf
• https://lists.gnupg.org/pipermail/gnupg-devel/2015-February/029544.html

I am removing the support for this proposal from NeoPG:

• There is very little adoption. Checking 642 email addresses from Debian maintainers shows only 8 PKA entries (and 22 TXT entries that are not PKA). Of these 8, several are expired keys (including the one from Simon Josefsson) or the URL is not reachable.
• There is no standard or draft of a standard, or other process towards standardization.
• All arguments against key retrieval via DNS apply (web bugs, lack of security in DNS, lack of control over DNS by users, difficulty of setting up DNS records, etc).
• There are standardized alternatives (DANE RFC7929, CERT RFC4398)

In the future, NeoPG will provide an API to extend key retrieval and trust evaluation, allowing such experimental protocols to be included in applications without tainting the core code base.