RB_REMOVE invoke RB_REMOVE_COLOR either when child is red or child is null. In the first case, RB_REMOVE_COLOR just changes the child to black and returns. WIth this change, RB_REMOVE handles that case, and drops the child argument to RB_REMOVE_COLOR, since that value is always null.
RB_REMOVE_COLOR is changed to remove a couple of unneeded tests, and to eliminate some deep indentation.
RB_ISRED is defined to combine a null check with a test for redness, to replace that combination in several places.