message: build_system: remove leftover EXTERNAL_DIR support
  This has not been used or tested for a long time, as far as I know, and
  likely won't work anymore.
diff: @@ -59,12 +59,6 @@ set(mavsdk_install_lib_dir ${lib_path})
  add_subdirectory(mavsdk)
  -if (DEFINED EXTERNAL_DIR AND NOT EXTERNAL_DIR STREQUAL "")
  - add_subdirectory(${EXTERNAL_DIR}/plugins
  - ${CMAKE_CURRENT_BINARY_DIR}/${EXTERNAL_DIR}/plugins)
  - include_directories(${EXTERNAL_DIR})
  -endif()
  -
  include(cmake/static_analyzers.cmake)
  if(BUILD_TESTS)
  @@ -73,11 +67,6 @@ if(BUILD_TESTS)
  add_subdirectory(integration_tests)
  - if (DEFINED EXTERNAL_DIR AND NOT EXTERNAL_DIR STREQUAL "")
  - add_subdirectory(${EXTERNAL_DIR}/integration_tests
  - ${CMAKE_CURRENT_BINARY_DIR}/${EXTERNAL_DIR}/integration_tests)
  - endif()
  -
  include(cmake/unit_tests.cmake)
  endif()
  