About the Matrix API left over

[Ducasse]

In Pharo in the past Array2D got some half finished Matrix API such as eigenvalue
and we should check and remove them.

Array2D is not a matrix (I was stupid to think about it that way).

[AhmedHamdiy]

@Ducasse I can work on this issue after finishing [AVL#29] addChild: is not in sync with its implementation

[AhmedHamdiy]

I didn't find anything related to matrix API in this repo. I think someone have solved this issue before.