Remove obsolete backend code

Remove obsolete backend code

- Remove obsolete code from lily.scm that was
used before the output backends took their current
form.

- Remove exported bindings from output-ps.scm; the
output backends no longer use a `public interface',
since all work is done with ly:outputter-dump-stencil,
which uses the modules directly.

- Remove the "catch-all" routines from the svg and socket
output backends; these are covered by the new procedure
in backend-library.scm