message: gitmodule: remove mavlink repo
  This is now fetched from cmake or manually.
diff: [submodule "mavsdk-proto"]
  path = proto
  url = https://github.com/mavlink/MAVSDK-Proto.git
  -[submodule "src/third_party/mavlink"]
  - path = src/third_party/mavlink
  - url = https://github.com/mavlink/mavlink.git
  