graphics/darktable: Remove empty PORTREVISION

The port failed to build with the following error:

graphics/darktable fails to build with:
/!\ darktable-4.4.1: Makefile errors /!\

PORTREVISION needs to be an integer >= 0

PR: 272628
Reported by: Philipp Ost <bsd@philippost.de>
Submitted by: Philipp Ost <bsd@philippost.de>