Import tree_reject from salsify_kafka

[ecopoesis]

Extract tree_reject from salsify_kafka so I can use it in salsify_graphql-server.

[jturkel]

Looks good. Might make sense to add some additional functionality in the future e.g. in place mutation (tree_reject!) or tree rejection via path (tree_reject(:a, :aa, :aaa)) but no need to do that now.

[jturkel]

The following would more clearly demonstrate how to call the function:

hash.tree_reject({
  a: {
    aa: :aaa
  }
})