Removing special case handling from vp9_tree_probs_from_distribution(),
tree_merge_probs(), and vp9_tokens_from_tree_offset() functions. Replacing
inter_mode_offset() function with macro INTER_OFFSET which is used now for
vp9_inter_mode_tree definition.

Change-Id: Iff75a1499d460beb949ece543389c8754deaf178