[pp] Remove unused printing tagging infrastructure.

Applications of it were not clear/unproven, it made printers more
complex (as they needed to be functors) and as it lacked examples it
confused some people.

The printers now tag unconditionally, it is up to the backends to
interpreted the tags.

Tagging (and indeed the notion of rich document) should be reworked in
a follow-up patch, so they are in sync, but this is a first step.

Tested, test-suite passes.

Notes:

- We remove the `Richprinter` module. It was only used in the
  `annotate` IDE protocol call, its output was identical to the normal
  printer (or even inconsistent if taggers were not kept manually in
  sync).

- Note that Richpp didn't need a single change. In particular, its
  main API entry point `Richpp.rich_pp` is not used by anyone.

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