Revert "Support generic default values"

This feature inadvertently introduced a breaking change (#144)

This reverts commits: feb6c4b2fe
b38a55331b.

schemars_derive/src/schema_exprs.rs

@@ -581,7 +581,7 @@ fn field_default_expr(field: &Field, container_has_default: bool) -> Option<Toke
             quote!(container_default.#member)
         }
         SerdeDefault::Default => quote!(<#ty>::default()),
-        SerdeDefault::Path(path) => quote!({ #path() as #ty }),
+        SerdeDefault::Path(path) => quote!(#path()),
     };
 
     let default_expr = match field.serde_attrs.skip_serializing_if() {