eliminate CX_ARGARRAY() and CX_ARGARRAY_set()

I added these earlier as a way of compiling the code under both 5.23.7 and
5.23.8, since the latter no longer has the argarray field in the context
struct. I didn't know at the time whether they were logically correct
under 5.23.8 - I just wanted the code to compile.

Having now properly fixed uplevel() to work on 5.23.8, and since uplevel
under 5.23.8 uses an entirely separate code path, only 5.23.7 and earlier
make use of CX_ARGARRAY() and _set(), so eliminate these macros and
revert back to the original usage (i.e. cx->blk_sub.argarray).