Fix fixed-scroll on Discover page (#13516) (#13557)

Some CSS changes were made in #10592
which caused the fixed-scroll directive to stop working in Discover.
Specifically the overflow rule which was added forced the width and
scrollWidth of the doc-container to be the same, so the fixed-scroll
directive would never add the scroll bar to the page. This PR simply
reverts the changes made in the above linked PR. I retested the original
issues in the latest version of Firefox and was unable to reproduce. It
seems that the original issues may have been fixed in Firefox itself.

Fixes #12416

src/core_plugins/kibana/public/dashboard/styles/index.less

@@ -219,4 +219,12 @@ dashboard-panel {
       height: auto;
     }
   }
+
+  /**
+   * 1. We want the doc-table-container to scroll only when embedded in a dashboard panel
+   */
+  .doc-table-container {
+    flex: 1 1 0; /* 1 */
+    overflow: auto; /* 1 */
+  }
 }

src/ui/public/doc_table/doc_table.less

@@ -12,14 +12,6 @@ doc-table {
   flex: 1 1 100%;
   flex-direction: column; /* 1 */
 
-  /**
-   * 1. Allow user to scroll table when its constrained by a fixed container height.
-   */
-  .doc-table-container {
-    flex: 1 1 0; /* 1 */
-    overflow: auto; /* 1 */
-  }
-
   .discover-table-datafield {
     white-space: pre-wrap;