Fix missing TreeTable column lines in IE8 (#12989)

Change-Id: Iddc6ecd5f455d28a43a67cdbf22cf81dbf3a6a5b

WebContent/VAADIN/themes/base/treetable/treetable.scss

@@ -26,7 +26,6 @@
 
 .#{$primaryStyleName} .v-table-row .v-table-cell-content,
 .#{$primaryStyleName} .v-table-row-odd .v-table-cell-content {
-    position: relative;
     z-index: 10;
 }