devnet-6: retire the xatu-experimental cannon (#32)

The per-devnet cannon stamped by mca-xatu-devnets in the platform repo
replaced this one: it coordinates against the devnet's own xatu server
and writes canonical data directly to clickhouse-raw's
glamsterdam-devnet-6 database. This chart still pointed at the retired
xatu-experimental pipeline on an image that cannot decode Gloas blocks,
so it derived nothing while hammering the beacon load balancer.

Author: samcm

kubernetes/devnet-6/xatu-cannon/values.yaml

@@ -1,5 +1,9 @@
+# The cannon for this devnet runs from the platform repo (mca-xatu-devnets
+# stamps a per-devnet cannon on the utility cluster) and writes canonical
+# data straight to clickhouse-raw. This chart fed the retired
+# xatu-experimental pipeline.
 xatu-cannon:
-  enabled: true
+  enabled: false
   fullnameOverride: xatu-cannon
   image:
     repository: ethpandaops/xatu