dylan: Remove old gwydion-dylan package.

The Gywdion Dylan compiler that was packaged here is old and this
package no longer builds. The Gwydion Dylan compiler is no longer
maintained, and has been replaced by the Open Dylan compiler which
is already packaged in Nix (opendylan).
Merge pull request #4047 from waywardmonkeys/remove-gwdyion-dylan

dylan: Remove old gwydion-dylan package.