message: cmake: remove the promise workaround
  Since we switched from the GNU stl to the LLVM stl for Android armeabi,
  we don't need the `DNO_PROMISES` workaround any longer.
diff: @@ -107,12 +107,6 @@ add_library(dronecore STATIC
  # other applications.
  if(ANDROID)
  set(lib_path "lib/android/${ANDROID_ABI}")
  -
  - # The old armeabi arch doesn't support promises yet with gcc 4.9.
  - if (ANDROID_ABI STREQUAL "armeabi")
  - add_definitions(-DNO_PROMISES)
  - endif()
  -
  elseif(IOS)
  set(lib_path "lib/ios")
  else()
  