Drop redundant custom MarkDecorator __eq__ implementation
Drop redundant custom MarkDecorator __eq__ implementation

This is already covered by attrs.

Also, the custom implementation returns False when the types don't
match, but it's better to return `NotImplemented`. attrs does this.

nicoddemus/pytest,Akasurde/pytest,nicoddemus/pytest,RonnyPfannschmidt/pytest,pytest-dev/pytest,markshao/pytest,The-Compiler/pytest,alfredodeza/pytest,The-Compiler/pytest