depend2.am: fix comments on verbosity of compilation rules

depend2.am: fix comments on verbosity of compilation rules

The situation and decisions described on those comments have become
quite outdated since the introduction of the silent-rules support.
Today, the general idea is to have nice, terse output if silent rules
are enabled, and complete, faithful, very verbose output if they are
not -- without trying to "massage" this verbose output in a more
pleasant form if that would cause complication in the affected code.

So it's better to just drop the obsolescent comments.

Note that we don't start simplifying the existing rules according
to this new philosophy; that will only be done when touching some
existing code (for the 'depend2.am' code, that will probably happen
on the master branch).

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>