Remove "array duck typing" section (#63)

This is covered throughout the document, from Use Cases and Scope to
Assumptions and Static Typing. Having a separate section on this
probably doesn't make sense.

Author: rgommers

spec/design_topics/array_ducktyping.md

@@ -8,7 +8,6 @@ Design topics & constraints
    eager_lazy_eval
    parallelism
    static_typing
-   array_ducktyping
    data_interchange
    accuracy
    device_support