Revert "backend-drm: add HDR_OUTPUT_METADATA definitions"

This reverts commit 69140640.

This is a follow-up change of b623fd2a ("drm-backend: stop parsing IN_FORMATS
blobs, use libdrm instead"). Weston now has a hard-requirement on libdrm
2.4.108, clean up remaining and unnecessary conditional code. Change 69140640
("backend-drm: add HDR_OUTPUT_METADATA definitions") is no longer needed
and stop including libdrm-updates.h from kms-color.c.

Signed-off-by: Luigi Santivetti <luigi.santivetti@imgtec.com>