Fix setting expectations on barely-generic functions

[asomers]

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