fix(button): Move theme-baseline() into base Sass.

BREAKING CHANGE: Removes button theme-baseline() mixin, moves mixin contents to base button Sass.

PiperOrigin-RevId: 317867315

Author: joyzhong

packages/button/src/mwc-button.scss

@@ -21,7 +21,6 @@ limitations under the License.
 @use '@material/theme';
 
 @include button.without-ripple();
-@include button.theme-baseline();
 
 .trailing-icon,
 .leading-icon, {