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-hpe/conf/layer.conf b/meta-hpe/conf/layer.conf
index 6ff79df..2b8f43b 100644
--- a/meta-hpe/conf/layer.conf
+++ b/meta-hpe/conf/layer.conf
@@ -7,7 +7,7 @@
 
 BBFILE_COLLECTIONS += "hpe-layer"
 BBFILE_PATTERN_hpe-layer := "^${LAYERDIR}/"
-LAYERSERIES_COMPAT_hpe-layer = "gatesgarth hardknott"
+LAYERSERIES_COMPAT_hpe-layer = "hardknott"
 
 # Provide a variable that points the base of the hpe layer.
 HPEBASE = '${@os.path.normpath("${LAYERDIR}/")}'
diff --git a/meta-hpe/meta-dl360poc/conf/layer.conf b/meta-hpe/meta-dl360poc/conf/layer.conf
index 5f85f7c..6e3d5c4 100644
--- a/meta-hpe/meta-dl360poc/conf/layer.conf
+++ b/meta-hpe/meta-dl360poc/conf/layer.conf
@@ -7,4 +7,4 @@
 
 BBFILE_COLLECTIONS += "dl360poc-layer"
 BBFILE_PATTERN_dl360poc-layer := "^${LAYERDIR}/"
-LAYERSERIES_COMPAT_dl360poc-layer = "gatesgarth hardknott"
+LAYERSERIES_COMPAT_dl360poc-layer = "hardknott"
diff --git a/meta-hpe/meta-gxp/conf/layer.conf b/meta-hpe/meta-gxp/conf/layer.conf
index 28f2ddc..4612c22 100644
--- a/meta-hpe/meta-gxp/conf/layer.conf
+++ b/meta-hpe/meta-gxp/conf/layer.conf
@@ -7,6 +7,6 @@
 BBFILE_COLLECTIONS += "gxp-layer"
 BBFILE_PATTERN_gxp-layer = "^${LAYERDIR}/"
 LAYERVERSION_gxp-layer = "1"
-LAYERSERIES_COMPAT_gxp-layer = "gatesgarth hardknott"
+LAYERSERIES_COMPAT_gxp-layer = "hardknott"
 
 GXPBASE = '${@os.path.normpath("${LAYERDIR}/")}'