doc: Remove UI Server documentation

* doc/Makefile.am: Remove uiserver.texi
* doc/gpgme.texi: Remove UI-Server mentions.
* doc/uiserver.texi: Removed.

This prepares the removal of UI Server from implementing
applications like Kleopatra.
The only user of the UI Server is GpgEX and even that
does not need it at all and is better served with process
calls.

• GnuPG-Bug-Id: T4030