Revert "Support generic default values"

This feature inadvertently introduced a breaking change (#144)

This reverts commits: feb6c4b
b38a553.

Author: GREsau

schemars/tests/default.rs

@@ -56,15 +56,3 @@ struct NotSerialize;
 fn schema_default_values() -> TestResult {
     test_default_generated_schema::<MyStruct>("default")
 }
-
-#[allow(dead_code)]
-#[derive(JsonSchema)]
-struct StructWithGenericDefaults {
-    #[serde(default = "Vec::new")]
-    a_vec: Vec<String>,
-}
-
-#[test]
-fn schema_with_generic_default_value() -> TestResult {
-    test_default_generated_schema::<StructWithGenericDefaults>("generic_default")
-}

schemars/tests/expected/generic_default.json

@@ -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() {