rb_tree: drop needless tests from rb_next, rb_prev

In RB_NEXT, when there is no RB_RIGHT node, the search must proceed
through the parent node.

There is code written to handle the case when the parent is non-NULL
and the current element is the left child of that parent. If you
assume that the current element is either the left child of its
parent, or the right child of its parent, but not both, then this test
is not necessary. Instead of assigning RB_PARENT(elm, field) to elm
when elm == RB_LEFT, removing the test has the code assign
RB_PARENT(elm, field) to elm when elm != RB_RIGHT. There's no need to
examine the RB_LEFT field at all.

This change removes that needless RB_LEFT test, and makes a similar
change to the RB_PREV implementation.

Reviewed by: alc
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35450

(cherry picked from commit 36447829aee545ad9eafbfff3a783ee58cfb28fd)