net: Remove the `pks` module.

  - The `pks` module provided a method to communicate with remote
    servers providing decryption and signing services.  However, there
    are several problems with this:

    - The protocol design does not fit into our vision of how a secret
      key store should work, and with the advent of the Sequoia
      Keystore (which does fit that vision) has become obsolete.

    - There is no downstream consumer of this API in the Sequoia
      project.

    - There are no integration tests, so the code is difficult to
      maintain.

    - The code requires porting to newer crates, and without
      integration tests and users this is difficult to do reliably.

    - The name unfortunately collides with the OpenPGP Key Server and
      associated protocol pks, an ancestor of hkp.

    - Arguably, this module should have gone into the sequoia-ipc
      crate and not into the sequoia-net crate in the first place.

  - Remove it.

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