minor: filter default bounds for introduce_named_type_parameter

Example
---
```rust
fn foo(bar: $0impl Sized) {}
```

**Before this PR**

```rust
fn foo<$0S: Sized>(bar: S) {}
```

**After this PR**

```rust
fn foo<$0S>(bar: S) {}
```

Author: A4-Tacks

crates/ide-assists/src/handlers/introduce_named_type_parameter.rs

@@ -48,7 +48,8 @@ pub(crate) fn introduce_named_type_parameter(
             )
             .for_impl_trait_as_generic(&impl_trait_type);
 
-            let type_param = make.type_param(make.name(&type_param_name), Some(type_bound_list));
+            let type_bound_list = non_default_bounds(&type_bound_list).then_some(type_bound_list);
+            let type_param = make.type_param(make.name(&type_param_name), type_bound_list);
             let new_ty = make.ty(&type_param_name);
 
             editor.replace(impl_trait_type.syntax(), new_ty.syntax());
@@ -63,6 +64,10 @@ pub(crate) fn introduce_named_type_parameter(
     )
 }
 
+fn non_default_bounds(bounds: &ast::TypeBoundList) -> bool {
+    bounds.bounds().collect_array().is_none_or(|[bound]| bound.syntax().text() != "Sized")
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -168,6 +173,15 @@ fn foo<
         );
     }
 
+    #[test]
+    fn replace_impl_default_bounds() {
+        check_assist(
+            introduce_named_type_parameter,
+            r#"fn foo(bar: $0impl Sized) {}"#,
+            r#"fn foo<$0S>(bar: S) {}"#,
+        );
+    }
+
     #[test]
     fn replace_impl_with_mut() {
         check_assist(