[button] Remove duplicated className entries

[silviuaavram]

• I have followed (at least) the PR section of the contributing guide.

Fixes #48189.

Button is sending both className and classes to ButtonRoot.

[mui-bot]

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Bundle size report

Bundle	Parsed size	Gzip size
@mui/material	▼-7B(0.00%)	▼-4B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against 2f58e1d

[Copilot]

Pull request overview

This PR fixes duplicated CSS class entries on the Material UI Button root element by avoiding passing the same utility classes through both className and the classes prop path (via ButtonBase).

Changes:

• Remove classes.root from the className passed to ButtonRoot, relying on the existing classes={classes} forwarding to apply utility classes once.

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[mnajdova]

I think we should omit classes.root when we propagate the classes prop instead. We should do that on all components using ButtonBase.

What I meant was doing this:

    <ButtonRoot
      ownerState={ownerState}
      className={clsx(contextProps.className, className, classes.root, positionClassName)}
      component={component}
      disabled={disabled || loading}
      focusRipple={!disableFocusRipple}
      focusVisibleClassName={clsx(classes.focusVisible, focusVisibleClassName)}
      ref={ref}
      internalNativeButton
      type={type}
      id={loading ? loadingId : idProp}
      {...other}
      // classes.root is already propagated in the className prop
      classes={{...classes, root: {}}}
    >

[ZeeshanTamboli]

I guess it's a regression since #25072. The issue is also present in v5: https://v5.mui.com/material-ui/react-button/.