htdocs: change link to trac
htdocs: removed files, they have been moved to the repo cppcheck-htdocs
Clean up .gitignore and remove the currently unneeded .gitattributes file.

[ci skip]
Update .gitignore.

Remove accidentally added file.

[ci skip]
Refactoring: Removed CheckNullPointer::nullPointerByCheckAndDeRef and implemented needed analysis in ValueFlow instead.