refresh row map after collection deletion

It refreshes customRowHeights which is used by windowed list to
calculate the right offsets.
Without this, customRowHeights remains same as before deletion, so windowed
list picks the offset based on stale data, which makes it use smaller
offset that makes headers overlap with other rows.

chrome/content/zotero/collectionTree.jsx

@@ -810,6 +810,7 @@ var CollectionTree = class CollectionTree extends LibraryTree {
 					this._removeRow(row);
 					this._removeRow(row - 1);
 				}
+				this._refreshRowMap();
 			}
 			// If there's an active filter, we can have a child matching filter be deleted
 			// which means the non-matching parent needs to be removed, so the tree is rebuilt