code_style: remove unnecessary fields

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/Views/Histories.axaml.cs

@@ -563,16 +563,12 @@ private void OnCommitListLayoutUpdated(object _1, EventArgs _2)
             SetCurrentValue(IsScrollToTopVisibleProperty, startY >= rowHeight);
 
             var clipWidth = dataGrid.Columns[0].ActualWidth - 4;
-            if (Math.Abs(_lastGraphStartY - startY) > 0.01 ||
-                Math.Abs(_lastGraphClipWidth - clipWidth) > 0.01 ||
-                Math.Abs(_lastGraphRowHeight - rowHeight) > 0.01)
-            {
-                _lastGraphStartY = startY;
-                _lastGraphClipWidth = clipWidth;
-                _lastGraphRowHeight = rowHeight;
-
+            var lastLayout = CommitGraph.Layout;
+            if (lastLayout == null ||
+                Math.Abs(lastLayout.StartY - startY) > 0.01 ||
+                Math.Abs(lastLayout.ClipWidth - clipWidth) > 0.01 ||
+                Math.Abs(lastLayout.RowHeight - rowHeight) > 0.01)
                 CommitGraph.Layout = new(startY, clipWidth, rowHeight);
-            }
         }
 
         private void OnScrollToTopPointerPressed(object sender, PointerPressedEventArgs e)
@@ -1657,9 +1653,6 @@ private async Task InteractiveRebaseWithPrefillActionAsync(ViewModels.Repository
                 await this.ShowDialogAsync(new ViewModels.InteractiveRebase(repo, on, prefill));
         }
 
-        private double _lastGraphStartY = 0;
-        private double _lastGraphClipWidth = 0;
-        private double _lastGraphRowHeight = 0;
         private bool _resizingAuthorColumn = false;
         private Cursor _resizingCursor = new Cursor(StandardCursorType.SizeWestEast);
     }