Remove TODOs from lossless encoder code.

histogram.c:
 - Verified (earlier) that there's low correlation between Red & Blue colors
   (particularly after applying Cross-color transform). The Bin based histogram
   merge, bins on three entropies viz literal, red & blue symbols. Removing
   either of blue or red increases the compression density. So keeping the bins
   for red & blue sybmols.
 - Keeping the compact bins method as-is. This way it's simpler to read.
huffman_encode.h: Added field comments for struct HuffmanTree and removed the TODO.

Change-Id: Ia76f7bc730079d1b3b644038c5d9931db3797f0e