Remove unnecessary base class and dead code in pylint.pyreverse.utils

[DudeNr33]

• Add a ChangeLog entry describing what your PR does.
• If it's a new feature, or an important bug fix, add a What's New entry in
  doc/whatsnew/<current release.rst>.
• Write a good description on what the PR does.
• If you used multiple emails or multiple names when contributing, add your mails
  and preferred name in script/.contributors_aliases.json

Type of Changes

	Type
✓	🔨 Refactoring

Description

Ref #6582
Closes the first bullet point from the list:
ASTWalker and LocalsVisitor were apparently moved from astroid.utils to pylint.pyreverse.utils some years ago.
Maybe originally there were also other subclasses of ASTWalker, but at least in pyreverse that is not the case.

Thus we can remove ASTWalker and some of the unused code in there and directly put the remaining stuff in LocalsVisitor.

Some remarks, as the diff is a bit hard to read imo:

• handler was always self -> removed handler and replaced references with self
• walk and leave were never called -> removed

[DudeNr33]

@Pierre-Sassoulas regarding your idea of user facing / dev related changes in #6632 (comment):
Do you think I should add a Changelog entry somewhere?

[coveralls]

Pull Request Test Coverage Report for Build 2394816447

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 95.513%

---

Totals
Change from base Build 2392933967:	0.09%
Covered Lines:	16221
Relevant Lines:	16983

---

💛 - Coveralls

[Pierre-Sassoulas]

@Pierre-Sassoulas regarding your idea of user facing / dev related changes in #6632 (comment): Do you think I should add a Changelog entry somewhere?

We'll do the switch for 2.15, let's keep on doing it like we were for 2.14.

[Pierre-Sassoulas]

LGTM, I think it can go in 2.14 even without changelog, it's an internal refactor, right ?

[DudeNr33]

Yes, this should be purely internal. At least I would not say the ASTWalker class is considered public API, and there is no plugin system for pyreverse.

I did a GitHub search and only found one instance where it was actually used, but that was just part of a demo talk in 2017.

I'll add a ChangeLog entry anyway just so that any potential users could find some information about it. Doesn't cost much and might prevent questions, although I don't really expect any problems.