meta-tegrademo/conf/layer.conf: update for testing DO NOT MERGE

Drop the meta-mender-tegra dynamic layer from meta-tegra-community.
Only needed during pre-upstream merge of the updates to
meta-mender-community/meta-mender-tegra and the corresponding drop
of the dynamic layer from meta-tegra-community.

Signed-off-by: Matt Madison <matt@madison.systems>