decode: Rework logic for text output of protocol decoder annotations

decode: Rework logic for text output of protocol decoder annotations

Fold the previous three code paths which formatted text output for
annotations into one code path, which inspects the flags that were
determined before the actual output, and then always formats the fields
in identical ways regardless of why a field is shown.

Remove the assert checks that were introduced in a previous commit. The
logic for the field selection was proven to be identical to the previous
behaviour.