Delete outdated pre-standard langauge description text.

Delete all the old content in Semantics.md, TextFormat.md, JS.md, BinaryFormat.md, and Modules.md, so that they just contain links to the relevant chapters of the official spec. The old content was out of date, in some places no longer accurate and in others no longer relevant, so having it in the tree is confusing for people who are searching for things and possibly missing the notices about being unmaintained at the tops of the files. All the content is still available in the git history in case anyone wants to do archeology.