layer.conf: drop layers not supported by master

dunfell / kirkstone are better supported in the correspondig branches.
langdale has reached EOL, it won't be supported anymore.

Signed-off-by: Stefano Babic <stefano.babic@swupdate.org>

conf/layer.conf

@@ -9,6 +9,6 @@ BBFILE_COLLECTIONS += "swupdate"
 BBFILE_PATTERN_swupdate := "^${LAYERDIR}/"
 BBFILE_PRIORITY_swupdate = "6"
 
-LAYERSERIES_COMPAT_swupdate = "dunfell kirkstone langdale mickledore nanbield"
+LAYERSERIES_COMPAT_swupdate = "mickledore nanbield"
 
 LAYERDEPENDS_swupdate = "openembedded-layer"