chore(util/rb_tree): cleanup code
chore(util/rb_tree): cleanup code

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

fpvandoorn/lean2,Kha/lean,rlewis1988/lean,levnach/lean,eigengrau/lean,levnach/lean,leanprover-community/lean,rlewis1988/lean,fgdorais/lean,digama0/lean,leanprover/lean,eigengrau/lean,dselsam/lean,leanprover/lean,fpvandoorn/lean,leodemoura/lean,digama0/lean,digama0/lean,levnach/lean,soonhokong/lean-osx,eigengrau/lean,fpvandoorn/lean,avigad/lean,dselsam/lean,dselsam/lean,rlewis1988/lean,soonhokong/lean-windows,fgdorais/lean,avigad/lean,Kha/lean,javra/lean,soonhokong/lean,javra/lean,dselsam/lean,avigad/lean,avigad/lean,fgdorais/lean,fpvandoorn/lean,eigengrau/lean,Kha/lean,soonhokong/lean,fgdorais/lean,fpvandoorn/lean,leanprover-community/lean,fpvandoorn/lean2,fgdorais/lean,soonhokong/lean-osx,avigad/lean,avigad/lean,levnach/lean,leanprover/lean,UlrikBuchholtz/lean,soonhokong/travis_test,UlrikBuchholtz/lean,eigengrau/lean,leodemoura/lean,c-cube/lean,Kha/lean,htzh/lean,htzh/lean,fgdorais/lean,leanprover/lean,leanprover-community/lean,soonhokong/lean-osx,johoelzl/lean,c-cube/lean,sp3ctum/lean,htzh/lean,johoelzl/lean,rlewis1988/lean,sp3ctum/lean,soonhokong/lean-windows,htzh/lean,soonhokong/travis_test,fpvandoorn/lean,htzh/lean,soonhokong/lean-windows,digama0/lean,leanprover-community/lean,leanprover-community/lean,sp3ctum/lean,fpvandoorn/lean2,fpvandoorn/lean2,digama0/lean,fpvandoorn/lean,johoelzl/lean,sp3ctum/lean,soonhokong/lean-windows,johoelzl/lean,soonhokong/lean-osx,javra/lean,c-cube/lean,c-cube/lean,soonhokong/lean-osx,fpvandoorn/lean2,soonhokong/lean,UlrikBuchholtz/lean,levnach/lean,soonhokong/lean,leodemoura/lean,leodemoura/lean,johoelzl/lean,UlrikBuchholtz/lean,javra/lean,leanprover/lean,c-cube/lean,leanprover/lean,rlewis1988/lean,Kha/lean,sp3ctum/lean,UlrikBuchholtz/lean,dselsam/lean,rlewis1988/lean,soonhokong/travis_test,soonhokong/travis_test,soonhokong/lean-windows,johoelzl/lean,Kha/lean,leodemoura/lean,digama0/lean,leanprover-community/lean,soonhokong/lean,javra/lean,leodemoura/lean,dselsam/lean