Guile already checks for the type of its objects before converting to C.  Thus
we don't have to do the check ourself.

The check is still made in `apply_to_guile' to avoid packing values unnecessary
if the procedure is not really a procedure.  The check is also made when
registering callbacks because of their delayed nature.

Change-Id: Id879c04d119884ad1226ed2495b880681125bccc