mesa-vpu: delete extension — fully covered by armbian-config

All remaining responsibilities of post_install_kernel_debs__3d moved to
armbian-configng's module_desktops, invoked via `module_desktops install
mode=build` during rootfs cache assembly:

  - Mesa runtime + Vulkan drivers + glmark2/mesa-utils/vulkan-tools:
    configng common.yaml mid tier (armbian/configng 3d8fff72).
  - panthor-gpu DT overlay: _module_desktops_rockchip_multimedia
    stage 2 (armbian/configng 41310eff).
  - amazingfated ppa:liujianfeng1994/rockchip-multimedia + pin at
    priority 1001 + rockchip-multimedia-config, libv4l-rkmpp,
    gstreamer1.0-rockchip, libwidevinecdm0 install on noble/rk3588/
    vendor: _module_desktops_rockchip_multimedia stage 1
    (armbian/configng 1329d6b4, follow-up in armbian/configng#887
    for the libv4l-0 pre-install ordering).

Caveat: configng sets up the PPA + pin and installs the multimedia
packages, but does not run a post-install `apt-get dist-upgrade
--allow-downgrades` to actively swap pre-installed chromium from
apt.armbian.com to the PPA's hardware-accelerated build. That swap
now happens on the user's first `apt upgrade` after boot (priority
1001 permits the downgrade-across-origins); shipped images carry
the apt.armbian.com chromium. If we want image-time swap back, a
follow-up in configng can add the dist-upgrade inside the same
noble/rk3588/vendor gate. The KDE-neon `apt-mark hold base-files`
and arm-noble/oracular `apt-mark hold libdav1d7` bracket calls are
dropped along with the dist-upgrade they were guarding.

No enable_extension or config references to mesa-vpu remain in the
build tree; no other callers to remove.

Author: igorpecovnik