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-supermicro/conf/layer.conf b/meta-supermicro/conf/layer.conf
index f46b374..d3943d3 100644
--- a/meta-supermicro/conf/layer.conf
+++ b/meta-supermicro/conf/layer.conf
@@ -7,5 +7,5 @@
 
 BBFILE_COLLECTIONS += "supermicro-layer"
 BBFILE_PATTERN_supermicro-layer := "^${LAYERDIR}/"
-LAYERSERIES_COMPAT_supermicro-layer := "gatesgarth hardknott"
+LAYERSERIES_COMPAT_supermicro-layer := "hardknott"