Have Phpstan Ignore Chart/Renderer/JpGraph

[oleibman]

This one class consumes a lot of space in Phpstan baseline. The problem is that it is an interface to Jpgraph, which is not maintained in Composer. This means that we have to disable tests involving this module, since we are dealing with very old code in our test suite. This means that we are very unlikely to do any work on this member, so the code error reports are more of a distraction than anything else. Remove them for now, restoring them if we ever solve this problem.

This is:

- [ ] a bugfix - [ ] a new feature - [ ] refactoring - [ ] additional unit tests 

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.