Remove Raspberry Pi 3 support

Remove Raspberry Pi 3 support

The raspberrypi3 machine template had only been retained to allow
anyone interested in using one with a telematics or cluster demo
image build to do so.  At this point the Pi 4 has been out long
enough that that seems no longer necessary, so removing the
support to avoid confusion and reduce clutter seems warranted.

Bug-AGL: SPEC-3985

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ib3770ba03771aff468f94d5bf6f2910fc01f7ee0
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26429
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>