Remove "missing data" and "portability" sections

The former is out of scope, the latter doesn't seem necessary
(whatever I can imagine saying there should already be covered
elsewhere).

Author: rgommers

spec/design_topics/index.rst

@@ -12,6 +12,4 @@ Design topics & constraints
    array_ducktyping
    data_interchange
    device support
-   missing_data
-   portability
    C_API