Remove attributes annotation of Builder `struct` (#3674)

The `struct` created for a Builder should not have attributes because
these structs only derive from `Debug`, `PartialEq`, `Clone` and
`Default`. None of these support attributes.

This becomes a problem if a plugin tries to add attributes to the
`metadata` field to configure the generated code for the `struct`. In
this case, the attributes will also be added to the `Builder` struct;
which is wrong.

## Motivation and Context

I'm customizing client code generation by overriding the method:

```kotlin
override fun symbolProvider(base: RustSymbolProvider): RustSymbolProvider
```

I override the `symbol.expectRustMetadata().additionalAttributes` value
and add extra values there. The generated code adds the correct
attributes for the `struct`, however, it also adds these attributes to
the `Builder`.

The `Builder` is restricted to deriving from `Debug`, `PartialEq`,
`Clone` and `Default` (see
[code](https://github.com/smithy-lang/smithy-rs/blob/a76dc184c2cf52d6027115f64fe78ab0c2a429e8/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/generators/BuilderGenerator.kt#L188)).

I believe this change was added by mistake in
https://github.com/smithy-lang/smithy-rs/pull/2222/

.


## Description

To solve the issue, I simply remove the offending line.

<!--- Describe your changes in detail -->

## Testing

I don't expect any existing code to break, given that this is a bug. It
only manifests when someone is customizing the generated code.


<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Co-authored-by: Gustavo Muenz <muenze@amazon.com>