Deprecate api bp2build of ndk_library and ndk_headers

To support export of NDK APIs in multi-tree, we added functionality to
generate Bazel targets of ndk related Soong module types. Since this use
case does not exist anymore, deprecate this to prevent bitrot

The removed code includes
- `ConvertWithApi2Build` implementation of these libraries, which
  generates the Bazel targets in the synthetic `api_bp2build` workspace
- (api) bp2build unit tests for these module types

Test: go build ./cc
Bug: 284029211
Change-Id: Id3278fa119e0ab87f31f39a3783197a81b655e43