redundant prefix .bootstrap-table .table > tbody > tr.groupBy + removal

the rigorous testing [on patch fiddle](https://github.com/wenzhixin/bootstrap-table/pull/3598#issuecomment-365179911) revealed the prefix `.bootstrap-table .table > tbody > tr.groupBy +` which has been initially applied just to follow the existing style is redundant &,  moreover, for this concrete case is harmful 'cause it brings the mess on selectors & leads to the fault of the whole patch scheme after all

as you can see https://jsfiddle.net/juliyvchirkov/jdwLp29e unprefixed rule achieves the goal as initially planned

src/extensions/group-by-v2/bootstrap-table-group-by.css

@@ -6,6 +6,6 @@
 
 }
 
-.bootstrap-table .table > tbody > tr.groupBy + tr.hidden + tr.detail-view {
+tr.hidden + tr.detail-view {
     display: none;
 }