CI: remove pmb:gpu-accel from custom_valid_options

This is not used anywhere in pmaports any more and it has been
deprecated for a while, so let's finally get rid of it.

Signed-off-by: Stefan Hansson <newbyte@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7847>

.ci/lib/apkbuild_linting.py

@@ -15,7 +15,6 @@ custom_valid_options = [
     "pmb:cross-native2",
     "pmb:drm",
     "pmb:generic-kernel",
-    "pmb:gpu-accel",  # deprecated
     "pmb:strict",
     "pmb:systemd",
     "pmb:systemd-never",