message: mission: remove unused dependency
  This fixes the build on Arch Linux without SUPERBUILD.
diff: -
  -hunter_add_package(jsoncpp)
  -find_package(jsoncpp CONFIG REQUIRED)
  -
  add_library(mavsdk_mission
  mission.cpp
  mission_impl.cpp
  @@ -20,8 +16,6 @@ set_target_properties(mavsdk_mission
  target_link_libraries(mavsdk_mission
  PUBLIC
  mavsdk
  - PRIVATE
  - JsonCpp::JsonCpp
  )
  target_include_directories(mavsdk_mission PUBLIC
  