Much faster CABAC RDO
Since RDO doesn't care about what order bit costs are calculated, merge sigmap and level coding into the same loop in RDO. This is bit-exact for 4x4dct but slightly incorrect for 8x8dct due to the sigmap containing duplicated contexts. However, the PSNR penalty of this is extremely small (~0.001db). Speed benefit is about 15% in 4x4dct and 30% in 8x8dct residual bit cost calculation at QP20. Overall encoding speed benefit is up to 5%, depending on encoding settings. Also remove an old unnecessary CABAC table that hasn't been used for years.
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