fix: table head does not align under firefox #579 (#627)

Author: sendya

components/vc-table/src/utils.js

@@ -9,7 +9,6 @@ const scrollbarMeasure = {
   top: '-9999px',
   width: '50px',
   height: '50px',
-  overflow: 'scroll',
 };
 
 export function measureScrollbar(direction = 'vertical') {