graphics/drm-fbsd13-kmod: Remove port

Remove the specific 13.X version of drm-kmod. There is no need for it
now that we have ports that directly tracks Linux numbering scheme.

Sponsored by: Beckhoff Automation GmbH & Co. KG
MFH: 2022Q3

(cherry picked from commit 70d3745d257f377df27431a8c18b5acfa3f973b5)