lib/rbtree: Remove dead case in rb_remove()

[Zhaoningx]

Describe the bug
I find some code that never been reached here in rb.c(lines 486~489) , the code screenshot as below:

let's do something firstly, Naming node that ready to be removed as A, and naming the biggest node of side 0 about A as B, so let me show you my point.
After my analysis, I think if the code can be reached here, It must meet the following conditions：
The first condition, the node A has two child, demo it such as(A is 30, B is 25, C is 23):

I think when B is red or C is black can reach the else{...} code (lines 486~489), but as you see, it is not satisfied the Red/black tree's principle.

The second condition, the node A just has one left child, demo it such as(A is 50, B is 45):

I think when A and B are both same color can reach the else{...} code (lines 486~489), but it's also not satisfied the rbtree's principle.

In my opinion, whatever we do, it can't be covered for the else{ ... } code except two conditions(with corrupt rbtree) above. So I think we should delete else{} code(lines 486~489).

Impact
It will makes a impact for tree's code coverage.

[Zhaoningx]

It has fixed, so closed.