LibGUI: Remove Model::sibling() since it's the same as index()

... I'm not sure what the idea was here, but since these functions do
the same thing, let's only have index().

Applications/FileManager/DirectoryView.cpp

@@ -325,7 +325,7 @@ void DirectoryView::update_statusbar()
 
     current_view().selection().for_each_index([&](auto& index) {
         auto& model = *current_view().model();
-        auto size_index = model.sibling(index.row(), GUI::FileSystemModel::Column::Size, model.parent_index(index));
+        auto size_index = model.index(index.row(), GUI::FileSystemModel::Column::Size, model.parent_index(index));
         auto file_size = model.data(size_index).to_i32();
         selected_byte_count += file_size;
     });

Libraries/LibGUI/ColumnsView.cpp

@@ -289,7 +289,7 @@ void ColumnsView::keydown_event(KeyEvent& event)
             auto old_index = selection().first();
             auto parent_index = model.parent_index(old_index);
             int row = old_index.row() > 0 ? old_index.row() - 1 : 0;
-            new_index = model.sibling(row, old_index.column(), parent_index);
+            new_index = model.index(row, old_index.column(), parent_index);
         } else {
             new_index = model.index(0, m_model_column, {});
         }
@@ -306,7 +306,7 @@ void ColumnsView::keydown_event(KeyEvent& event)
             auto old_index = selection().first();
             auto parent_index = model.parent_index(old_index);
             int row = old_index.row() + 1;
-            new_index = model.sibling(row, old_index.column(), parent_index);
+            new_index = model.index(row, old_index.column(), parent_index);
         } else {
             new_index = model.index(0, m_model_column, {});
         }

Libraries/LibGUI/Model.cpp

@@ -73,16 +73,6 @@ ModelIndex Model::index(int row, int column, const ModelIndex&) const
     return create_index(row, column);
 }
 
-ModelIndex Model::sibling(int row, int column, const ModelIndex& parent) const
-{
-    if (!parent.is_valid())
-        return index(row, column, {});
-    int row_count = this->row_count(parent);
-    if (row < 0 || row > row_count)
-        return {};
-    return index(row, column, parent);
-}
-
 bool Model::accepts_drag(const ModelIndex&, const StringView&)
 {
     return false;

Libraries/LibGUI/Model.h

@@ -81,7 +81,6 @@ public:
     virtual void update() = 0;
     virtual ModelIndex parent_index(const ModelIndex&) const { return {}; }
     virtual ModelIndex index(int row, int column = 0, const ModelIndex& parent = ModelIndex()) const;
-    virtual ModelIndex sibling(int row, int column, const ModelIndex& parent) const;
     virtual bool is_editable(const ModelIndex&) const { return false; }
     virtual void set_data(const ModelIndex&, const Variant&) { }
     virtual int tree_column() const { return 0; }

Libraries/LibGUI/TreeView.cpp

@@ -288,7 +288,7 @@ void TreeView::paint_event(PaintEvent& event)
 
             if (column_index != tree_column) {
                 Gfx::IntRect cell_rect(horizontal_padding() + x_offset, rect.y(), column_width, item_height());
-                auto cell_index = model.sibling(index.row(), column_index, index.parent());
+                auto cell_index = model.index(index.row(), column_index, index.parent());
 
                 if (auto* delegate = column_data(column_index).cell_painting_delegate.ptr()) {
                     delegate->paint(painter, cell_rect, palette(), model, cell_index);