Always have SATD based pruning in xCheckRDCostMerge2Nx2N

It is observed that under CTC (CIIP is enabled), always having SATD based pruning in xCheckRDCostMerge2Nx2N shows a bit better tradeoff. Moreover, it is easier for further refactoring.

The results relative to VTM-19 anchor are as follows.

Random access Main10
			Over vtm-19
	Y	U	V	EncT	DecT
Class A1	-0.01%	-0.03%	0.02%	98%	97%
Class A2	-0.02%	-0.07%	0.04%	98%	97%
Class B	0.01%	0.01%	-0.02%	99%	99%
Class C	-0.01%	0.08%	0.03%	100%	100%
Class E
Overall	0.00%	0.00%	0.01%	99%	98%
Class D	-0.06%	0.04%	0.13%	100%	99%
Class F	0.00%	-0.01%	-0.02%	100%	99%
			Low delay B Main10
			Over vtm-19
	Y	U	V	EncT	DecT
Class A1
Class A2
Class B	-0.01%	0.13%	-0.09%	97%	94%
Class C	0.00%	0.06%	0.01%	99%	96%
Class E	0.06%	-0.17%	0.38%	99%	96%
Overall	0.01%	0.03%	0.06%	98%	95%
Class D	0.01%	-0.18%	0.07%	100%	102%
Class F	0.09%	0.12%	-0.47%	99%	99%
			Low delay P Main10
			Over vtm-19
	Y	U	V	EncT	DecT
Class A1
Class A2
Class B	0.01%	-0.05%	-0.01%	97%	94%
Class C	-0.01%	-0.31%	-0.09%	99%	95%
Class E	-0.10%	0.44%	-0.04%	98%	95%
Overall	-0.02%	-0.01%	-0.04%	98%	95%
Class D	0.05%	-0.75%	0.28%	100%	102%
Class F	0.04%	-0.03%	-0.50%	99%	96%