Cleanup

- move the development documentation to `devel-doc` subpackage
- removed obsolete BuildRequires, not needed anymore:
    - docbook-xsl-stylesheets
    - doxygen
    - gcc-c++
    - libxslt perl-Bootloader
    - perl-gettext
    - perl-XML-Writer
    - sgml-skel
    - swig
    - update-alternatives
    - libtool
    - yast2-installation
    - yast2-packager
    - yast2-perl-bindings
    - yast2-pkg-bindings
    - yast2-storage
    - yast2-testsuite
- removed empty `agents/Makefile.am`
- 3.0.1