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-intel-openbmc/conf/layer.conf b/meta-intel-openbmc/conf/layer.conf
index 3b53a7f..f082caf 100644
--- a/meta-intel-openbmc/conf/layer.conf
+++ b/meta-intel-openbmc/conf/layer.conf
@@ -8,5 +8,5 @@
 BBFILE_COLLECTIONS += "intel"
 BBFILE_PATTERN_intel = "^${LAYERDIR}/"
 LAYERVERSION_intel = "1"
-LAYERSERIES_COMPAT_intel = "gatesgarth hardknott honister"
+LAYERSERIES_COMPAT_intel = "hardknott honister"
 LAYERDEPENDS_intel = "phosphor-layer"