If declosurefy removes all of a where clause's predicates, then it
should remove the whole where clause.  This results in some functions
will no longer need to use generic Expectations structs.

If the function's generic parameters appeared in neither input nor
return type, the Expectations object would not be parameterized on the
functions generic parameters.  This wasn't really by design; it was
actually an artifact of Mockall's second incarnation, which used a
macro_rules! macro to generate the mock objects.

Fixing the problem is easy.  It's a matter of removing the code that
tried to guess the generic parameters based on the functions arguments
and return types.

Fixes #269