Clean input group flex basis (#28785)

scss/_input-group.scss

@@ -16,10 +16,7 @@
   > .custom-select,
   > .custom-file {
     position: relative; // For focus state's z-index
-    flex: 1 1 auto;
-    // Add width 1% and flex-basis auto to ensure that button will not wrap out
-    // the column. Applies to IE Edge+ and Firefox. Chrome does not require this.
-    width: 1%;
+    flex: 1 1 0%;
     margin-bottom: 0;
 
     + .form-control,