remove deprecated stuff from graphs

[jfraymond]

Goal: remove all the code of the graph module that has a deprecation warning referring to a ticket closed for at least one year.
These tickets are:

• deprecate import of all_max_clique, max_clique and clique_number in global namespace #26200 deprecate import of all_max_clique, max_clique and clique_number in global namespace
• deprecate parameter copy in networkx_graph #27491 deprecate parameter copy in networkx_graph
• Graphs: DFS and broken distance-parameter #19227 Graphs: DFS and broken distance-parameter
• Graphs: canonical_label() and several errors #19517 Graphs: canonical_label() and several errors
• make LinearExtensions an iterator #25864 make LinearExtensions an iterator

Note that the current ticket completely deletes the file graphs/linearextensions.py.

Component: graph theory

Keywords: deprecation

Author: Jean-Florent Raymond

Branch/Commit: b1f507f

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/29518

[jfraymond]

Author: Jean-Florent Raymond

[jfraymond]

comment:1

On my machine the doctests for the graphs folder pass and the doc builds.

[fchapoton]

comment:2

Looks good.

Could you please list in the ticket description field here the trac numbers of the tickets that introduced these deprecations ?

[fchapoton]

Reviewer: Frédéric Chapoton

[fchapoton]

comment:4

ok, merci !

[vbraun]

Changed branch from u/gh-jfraymond/remove-deprecated-stuff-in-graphs to b1f507f