Make Dynkin diagrams coming from a Cartan type immutable and better respect edge deletion

[tscrim]

So we don't get behavior like this:

sage: d = DynkinDiagram(['B',3])
sage: d.edges()
[(1, 2, 1), (2, 1, 1), (2, 3, 2), (3, 2, 1)]
sage: d
O---O=>=O
1   2   3
B3
sage: d.delete_edge(1,2)
sage: d.edges()
[(2, 1, 1), (2, 3, 2), (3, 2, 1)]
sage: d    # This iswrong
O---O=>=O
1   2   3
B3
sage: CartanMatrix(d)   # So is this
[ 2 -1  0]
[ 0  2 -1]
[ 0 -2  2]

CC: @sagetrac-sage-combinat @nthiery @sagetrac-jakobkroeker

Component: combinatorics

Keywords: dynkin diagrams

Stopgaps: wrongAnswerMarker

Issue created by migration from https://trac.sagemath.org/ticket/15739

[sagetrac-jakobkroeker]

Stopgaps: wrongAnswerMarker