C4 fraction option should be removed

[rml599gh]

CABLE's photosynthesis code was originally written to accommodate vegetation types which were a mix of c3 and c4 plants. In practice, I don't think this has been used for many years as all vegetation types have been set up as all c3 or all c4.
The Medlyn based photosynthesis code, which is now an option, does not appear to allow for mixed types.
It would make the code cleaner if mixed c3/c4 types were no longer supported.