Remove unused private attribute from KProve

[tothtamas28]

Eliminates a (package level) circularity between pyk.kcfg and pyk.ktool.

[tothtamas28]

pydeps -o k.png -T png --no-show --cluster --max-bacon 2 --max-module-depth 2 pyk/src/pyk

Before:

After:

[ehildenb]

I find it weird that ktool is included into kcfg, but I guess that's because KCFGExplore uses ktool. I'd like to have kcfg be a bit more "pure" of a datastructure, so maybe we move the explore.py and exploration.py modules into proof instead, since that's really where they are used.

We have some other work to remove the use of KPrint from kcfg class too: #4808