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-evb/meta-evb-aspeed/meta-evb-ast2600/conf/layer.conf b/meta-evb/meta-evb-aspeed/meta-evb-ast2600/conf/layer.conf
index b3ef55c..277e712 100644
--- a/meta-evb/meta-evb-aspeed/meta-evb-ast2600/conf/layer.conf
+++ b/meta-evb/meta-evb-aspeed/meta-evb-ast2600/conf/layer.conf
@@ -8,4 +8,4 @@
 BBFILE_COLLECTIONS += "meta-evb-ast2600"
 BBFILE_PATTERN_meta-evb-ast2600 = "^${LAYERDIR}/"
 
-LAYERSERIES_COMPAT_meta-evb-ast2600 = "gatesgarth hardknott honister"
+LAYERSERIES_COMPAT_meta-evb-ast2600 = "hardknott honister"