Remove unused CHPL_RT_MD types:

- CHAPEL_CODE
- GC_HEAP
- GC_SPACE_POINTER
- FLAG_SERIAL
Merge pull request #116 from sungeunchoi/test

Remove unused memory descriptor types

[reviewed by Tom]

This commit removes the following unused memory descriptor types:

- CHAPEL_CODE
- GC_HEAP
- GC_SPACE_POINTER
- FLAG_SERIAL

There are a few remaining unused types, but they seemed like they may be useful in the future (or were possibly not used on accident) as they are related to other descriptors that are used, so I left them in there.