Bug 33991: Remove unused features

For simplification, we remove some unused features. Those features can
still be reimplemented externally if needed.

For example, the "rbm rpm" command can be replaced with
"rbm build --step rpm" and a definition of an rpm step in rbm.conf.

We remove the following options:
 - version_command
 - rpm
 - rpmbuild
 - srpm
 - rpmspec
 - rpm_rel
 - debian_create
 - deb
 - deb_src
 - use_pbuilder
 - debian_revision

And the following commands:
 - rpmspec
 - rpm
 - srpm
 - deb-src
 - deb
 - pkg
 - publish