ci: drop retired mqtt-bridge-implementation-flex trigger name

The rename to observer-firmware is verified: the rename's own push event
ran both workflows on the new name (branch creation bypasses path
filters), sync-flasher-content pushed to the flasher repo, and the
production publish completed on observer-firmware. The transition dual
trigger is no longer needed.

Also update the MQTT_INTERNALS.md prose reference to the old branch name.

Workflow+docs-only commit: pushing this triggers no build.

.github/workflows/build-observer-firmwares.yml

@@ -7,10 +7,6 @@ on:
   workflow_dispatch:
   push:
     branches:
-      # Both names during the flex -> observer-firmware rename: GitHub does not
-      # rewrite branch filters on rename, so the old name is kept until a
-      # publish is verified on the new one (then it gets dropped).
-      - mqtt-bridge-implementation-flex
       - observer-firmware
     # Only rebuild firmware when something that affects the binaries changes.
     # Docs, the changelog, the changelog generator, CI files, and repo-meta

.github/workflows/sync-flasher-content.yml

@@ -14,10 +14,6 @@ on:
   workflow_dispatch:
   push:
     branches:
-      # Both names during the flex -> observer-firmware rename: GitHub does not
-      # rewrite branch filters on rename, so the old name is kept until a
-      # publish is verified on the new one (then it gets dropped).
-      - mqtt-bridge-implementation-flex
       - observer-firmware
     paths:
       - '**.md'