meta-agl-core: update poky-agl distro configuration

meta-agl-core: update poky-agl distro configuration

Changes to match current poky:
- Remove SDK_VERSION definition since the one inherited from
  poky.conf should be fine
- Replace explicit systemd enabling with use of INIT_MANAGER variable
- Moved some of the DISTRO_FEATURES variables together to improve
  readability
- Remove gstreamer1.0* PREFERRED_VERSION overrides, as they no longer
  seem required
- Remove PREFERRED_PROVIDER_jpeg as it no longer is required

Note that these are in part cleanup to match poky dunfell, and also
to reduce the amount of changes required to support current upstream
poky master.

Bug-AGL: SPEC-3819

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I540dd97eef147201446021dbe0bda5289e34a18f
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26100
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>