remove ndk_helper #2251

afaik this was declared obsolete in 2017, requires (for at least some of its functionality) a .java file we don't ship, and is already included in the ndk-samples "teapot" sample which is where this code came from in the first place.

it seems to be a mistake that it was ever included in the ndk, is trivially available from ndk-samples if anyone does want it (a point which we should mention in the release notes), and should be removed.

(the only docs reference i can find to ndk_helper seems to be https://developer.android.com/ndk/samples/sample_teapot which should perhaps itself be removed/moved to the ndk-samples github project?)