reverted changes in commit 96109d3 which seemed to break conditional classes in striped tables (ignoring the comment above that the previous rules were needed for just this purpose)

fixes issue twbs/bootstrap#11728

less/mixins.less

@@ -414,24 +414,24 @@
 .table-row-variant(@state; @background) {
   // Exact selectors below required to override `.table-striped` and prevent
   // inheritance to nested tables.
-  .table {
-    > thead,
-    > tbody,
-    > tfoot {
-      > tr > .@{state},
-      > .@{state} > td,
-      > .@{state} > th {
-        background-color: @background;
-      }
+  .table > thead > tr,
+  .table > tbody > tr,
+  .table > tfoot > tr {
+    > td.@{state},
+    > th.@{state},
+    &.@{state} > td,
+    &.@{state} > th {
+      background-color: @background;
     }
   }
 
   // Hover states for `.table-hover`
   // Note: this is not available for cells or rows within `thead` or `tfoot`.
-  .table-hover > tbody {
-    > tr > .@{state}:hover,
-    > .@{state}:hover > td,
-    > .@{state}:hover > th {
+  .table-hover > tbody > tr {
+    > td.@{state}:hover,
+    > th.@{state}:hover,
+    &.@{state}:hover > td,
+    &.@{state}:hover > th {
       background-color: darken(@background, 5%);
     }
   }