[ffigen] Remove FfiNativeConfig from Global and Func

[liamappelbe]

It's only used for its bool enabled field, and it's confusing to have part of the config copied into these objects when it's also available from the Context.

Yak shaving for #2419

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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[coveralls]

coverage: 82.9% (-0.01%) from 82.913%
when pulling ba5a23f on ffigen_native_config
into 813edba on main.