fix(telemetry): keep category names on Overlay Traffic bargauge

The "Overlay Traffic by Category (Bytes In)" bargauge names each bar by
category via 22 byName field-override displayNames (Transactions, Proposals,
...). The legend migration had also added a generic
${__field.labels.series} [...] Display name, which competed with those
curated names. Remove the generic Display name (and its now-unneeded
empty-bracket cleanup transform) from this panel so the category names stand;
this panel's identity is the traffic category, not the node/perf-iac grouping.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker/telemetry/grafana/dashboards/network-traffic.json

@@ -445,7 +445,6 @@
       ],
       "fieldConfig": {
         "defaults": {
-          "displayName": "${__field.labels.series} [${__field.labels.resource_group}] [${__field.labels.iac_group}]",
           "unit": "Bps"
         },
         "overrides": [
@@ -714,16 +713,7 @@
             ]
           }
         ]
-      },
-      "transformations": [
-        {
-          "id": "renameByRegex",
-          "options": {
-            "regex": "/\\s*\\[\\]/g",
-            "renamePattern": ""
-          }
-        }
-      ]
+      }
     },
     {
       "title": "Duplicate Traffic (Wasted Bandwidth)",