[loopfilter] remove filter_length_internal
[loopfilter] remove filter_length_internal

filter_length_internal seems redundant in the current implementation of
deblocking filter.

Change-Id: I40ada51857556c38cca56da33e17d0c2c82b8fc1

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom