Remove pres and codes from the document

Following #64 I propose to *remove* the ignored elements from the Check class as soon as possible. This  simplifies all the later search routines.

~~I can’t figure out, how to remove nodes with `data-proofer-ignore` in the `remove_ignored` procedure.  This would be a better place to do it too.~~
If user says “ignores this element”, we must obey. An ignored favicon doesn’t exist for us, so this test should fail, no matter how other attributes are.