This reverts commit 1dad75e6.

Conflicts:
	configure

In Dedukti v2.6, we can now declare term as injective so the user can
handle higher-order logic by adding injective rewrite rules on term.

The main motivation is to ease integration into higher-order logic
which would orhterwise require fol.term to be a definable
symbol (which poses injectivity problems).

This change requires -coc, and the latest (v2.5.1) versions of Dedukti
and Meta Dedukti (because -coc was not present before in Meta
Dedukti). fol.term is now defined as the identity over types so it is
not mandatory to remove it from the developments but fol.type needs to
be replaced by Type since Dedukti does not allow kind-level
definitions.

A new bug has been discovered (#21276). It has been triggered by the
eqcert.match_f_equal_goal function and has required to remove the
eqcert.f_equal certificate transformer.

This corresponds to my presentation at
LSV (http://deducteam.gforge.inria.fr/seminars/170302.html). The
manual is still incomplete.