[DataGrid] Fix last separator not being hidden when grid is scrollable (

@KenanYusuf) (#15551)

Co-authored-by: Kenan Yusuf <[email protected]>

packages/x-data-grid/src/hooks/features/columnHeaders/useGridColumnHeaders.tsx

@@ -456,7 +456,7 @@ export const useGridColumnHeaders = (props: UseGridColumnHeadersProps) => {
           fields={headerInfo.fields}
           colIndex={headerInfo.colIndex}
           depth={depth}
-          isLastColumn={headerInfo.colIndex === visibleColumns.length - headerInfo.fields.length}
+          isLastColumn={index === visibleColumnGroupHeader.length - 1}
           maxDepth={headerGroupingMaxDepth}
           height={dimensions.groupHeaderHeight}
           hasFocus={hasFocus}