Removing 8-bit pipeline from encoder and decoder

AOM Common Test Conditions use 16-bit encoding pipeline for both encoding and decoding. This is defined by --use-16bit-internal parameter, which default value enforces using 16-bit pipeline (i.e. 16-bit frame buffers) for any content.

It was decided by the CWG to remove the 8-bit path from the codebase and only leave the 16-bit path. This would require:

• removing 8-bit versions of the C functions that have both 8-bit and 16-bit variants
• removing the corresponding 8-bit SIMD functions
• removing corresponding unit tests
• removing the code that does switching between 8-bit and 16-bit paths