RVAL_TYPE_ASSOC is unused, kill it.

I stumbled upon this while trying to understand the various RVAL types, I never got to grasp its intent so I killed it. ;-) All test suite seems to run undisturbed.

Please note that I used ProgrammingError() in rlist.c when rval is of unknown type, while the way used so far is to CfDebug() and return NULL. I thought it's better, let me know if it's not preferred.