type: remove unnecessary code (#43439)

components/button/button-group.tsx

@@ -1,9 +1,9 @@
 import classNames from 'classnames';
 import * as React from 'react';
+import warning from '../_util/warning';
 import { ConfigContext } from '../config-provider';
 import type { SizeType } from '../config-provider/SizeContext';
 import { useToken } from '../theme/internal';
-import warning from '../_util/warning';
 
 export interface ButtonGroupProps {
   size?: SizeType;
@@ -13,7 +13,7 @@ export interface ButtonGroupProps {
   children?: React.ReactNode;
 }
 
-export const GroupSizeContext = React.createContext<SizeType | undefined>(undefined);
+export const GroupSizeContext = React.createContext<SizeType>(undefined);
 
 const ButtonGroup: React.FC<ButtonGroupProps> = (props) => {
   const { getPrefixCls, direction } = React.useContext(ConfigContext);