Remove unique from index (tree_rev_descendant_id, tree_rev_id)

tree_rev_id is a primary key which is already unique, it would cause
extra overhead to enforce unique on (tree_rev_descendant_id, tree_rev_id)

Change-Id: I60e220daed763f8c28deacebc007c5e86dc39f58
Merge "Remove unique from index (tree_rev_descendant_id, tree_rev_id)"