Removing custom SubtleButton-styles (#41900)

## Summary of the Pull Request

We have migrated `SubtleButtonStyle` to Windows App SDK, so now that we
migrated to 1.8 we no longer need custom implementations and can just
refer to the built-in styles.

This PR:
- Removed custom SubtleButton styles
- Added the style to the pin button in Peek so it's more inline with the
Photos app design

## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/settings-ui/Settings.UI/SettingsXAML/App.xaml

@@ -13,7 +13,6 @@
                 <ResourceDictionary Source="/SettingsXAML/Controls/KeyVisual/KeyCharPresenter.xaml" />
                 <ResourceDictionary Source="/SettingsXAML/Controls/TitleBar/TitleBar.xaml" />
                 <ResourceDictionary Source="/SettingsXAML/Styles/TextBlock.xaml" />
-                <ResourceDictionary Source="/SettingsXAML/Styles/Button.xaml" />
                 <ResourceDictionary Source="/SettingsXAML/Styles/InfoBadge.xaml" />
                 <ResourceDictionary Source="/SettingsXAML/Themes/Colors.xaml" />
                 <ResourceDictionary Source="/SettingsXAML/Themes/Generic.xaml" />