Eliminate support for direct_8x8_inference=0
The benefit in the most extreme contrived situation was at most 0.001db PSNR, at the cost of slower decoding. As this option was basically useless, it was a waste of code and prevented some other useful optimizations. Remove some unused mc code related to sub-8x8 partitions. Small deblocking speedup when p4x4 is used. Also remove unused x264_nal_decode prototype from x264.h.
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