treewide: remove gategarth from layer-support

We've typically kept these LAYERSERIES_COMPAT to 2 releases: the current
and the upcoming.  Remove 'gatesgarth' is it is now 2 releases back.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5e812a94fed1738898af75c0fdee81996a5bbf20
diff --git a/meta-arm/conf/layer.conf b/meta-arm/conf/layer.conf
index d111b24..102a0cc 100644
--- a/meta-arm/conf/layer.conf
+++ b/meta-arm/conf/layer.conf
@@ -9,4 +9,4 @@
 BBFILE_PATTERN_arm = "^${LAYERDIR}/"
 LAYERVERSION_arm = "1"
 
-LAYERSERIES_COMPAT_arm = "gatesgarth hardknott"
+LAYERSERIES_COMPAT_arm = "hardknott"