Revert "Term: include a function to print terms"

Such printer is already in Termops

This reverts commit 5d6106a075b79abbb92b03bbca7b13a517cf4925.
universes_of_constant: do a proper set union of body and type univs

Before the union was performed as a UContext.t union, that
concatenates the instances arrays, while one wants to avoid
duplicates.

We also assert that polymorphic constants have all constraints
in the constant_body (field const_universes), since the extra body
univs (stored in the opaque tables) are just for regular constants
processed asynchronously.
include test-suite/coqchk in the summary log