Blender Git Loki

Cleanup: remove (unused) RNA update cache

Introduced in 2011 in rB6a392e8cb505, it was disabled again soon after
in rBb062056c05a3 and traces to it partly removed in rB21744217cea9.

Now remove completely.

quote @sergey:
We shouldn't be having partially working unused code.
If we ever need some sort of update cache it would need to have clear
design first, and the code could be resurrected from history if needed.

Differential Revision: https://developer.blender.org/D7432