telemetry: Deprecate prometheus.zeek policy script

With Cluster::Node$metrics_port being optional, there's not really
a need for the extra script. New rule, if a metrics_port is set, the
node will attempt to listen on it.

Users can still redef Telemetry::metrics_port *after*
base/frameworks/telemetry was loaded to change the port defined
in cluster-layout.zeek.

(cherry picked from commit bf9704f339)

testing/btest/coverage/bare-mode-errors.test

@@ -9,4 +9,4 @@
 #
 # @TEST-EXEC: test -d $DIST/scripts
 # @TEST-EXEC: for script in `find $DIST/scripts/ -name \*\.zeek`; do zeek -b --parse-only $script >>errors 2>&1; done
-# @TEST-EXEC: TEST_DIFF_CANONIFIER="grep -v -e 'load-balancing.zeek.*deprecated script loaded' | $SCRIPTS/diff-remove-abspath | $SCRIPTS/diff-sort" btest-diff errors
+# @TEST-EXEC: TEST_DIFF_CANONIFIER="grep -v -e 'load-balancing.zeek.*deprecated script loaded' | grep -v -e 'prometheus.zeek.*deprecated script loaded' | $SCRIPTS/diff-remove-abspath | $SCRIPTS/diff-sort" btest-diff errors

testing/btest/coverage/test-all-policy-cluster.test

@@ -9,7 +9,7 @@
 # @TEST-EXEC: CLUSTER_NODE=logger-1 zeek %INPUT
 # @TEST-EXEC: CLUSTER_NODE=proxy-1 zeek %INPUT
 # @TEST-EXEC: CLUSTER_NODE=worker-1 zeek %INPUT
-# @TEST-EXEC: TEST_DIFF_CANONIFIER='grep -v "load-balancing.zeek.*deprecated script" | $SCRIPTS/diff-remove-abspath' btest-diff .stderr
+# @TEST-EXEC: TEST_DIFF_CANONIFIER='grep -v "load-balancing.zeek.*deprecated script" | grep -v "prometheus.zeek.*deprecated script" | $SCRIPTS/diff-remove-abspath' btest-diff .stderr
 
 @load base/frameworks/cluster
 @load misc/loaded-scripts

testing/btest/scripts/policy/frameworks/telemetry/prometheus.zeek

@@ -55,7 +55,6 @@ done
 @TEST-END-FILE
 
 @load policy/frameworks/cluster/experimental
-@load policy/frameworks/telemetry/prometheus
 @load base/frameworks/telemetry
 
 # So the cluster nodes don't terminate right away.