Bugfix/issue 1078 recursive funs

#### Summary:

Remove duplicate generation of functors for forward declarations of user-defined functions.

#### Intended Effect:

See summary.

#### How to Verify:

The new syntax-only test model `src/test/test-models/syntax-only/functions-fwd-ref.stan` would not compile before because of duplicate definitions of functor.    It compiles now.

#### Side Effects:

None.

#### Documentation:

None.  Behavior now matches documentation.

#### Reviewer Suggestions:

Anyone.