Delete the settings file for the StyleCop we do not use

src/Settings.StyleCop is configuration for the old StyleCop (the file declares
Version="105", the 4.x/Visual StyleCop format). The build uses
StyleCop.Analyzers, which reads only stylecop.json - and never sees this file,
since it is not in AdditionalFiles and is referenced from no project, ruleset,
solution or workflow. Its last change was the /Source to /src rename seven
years ago.

It is also actively misleading: it disables ElementsMustBeDocumented,
FileMustHaveHeader, FileHeaderMustShowCopyright and
UsingDirectivesMustBePlacedWithinNamespace - all of which the code follows and
the build enforces today.

Build diagnostics and all 117 tests are unchanged without it.

Author: NikolayIT