Drop the write-only lastTooltipSegment field

The decompiler text view's lastTooltipSegment was assigned on hover and
cleared on close but never read anywhere, so it tracked nothing. Remove
the field and its three assignments.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ILSpy/TextView/DecompilerTextView.axaml.cs

@@ -85,7 +85,6 @@ namespace ILSpy.TextView
 		// TODO: drop this field and use Editor.ScrollToVerticalOffset once #594 ships and we bump the package.
 		ScrollViewer? editorScrollViewer;
 		DisplaySettings? currentDisplaySettings;
-		ReferenceSegment? lastTooltipSegment;
 		ReferenceSegment? lastRightClickedSegment;
 		IReadOnlyList<IContextMenuEntryExport> contextMenuEntries = Array.Empty<IContextMenuEntryExport>();
 		readonly Popup richPopup;
@@ -809,7 +808,6 @@ namespace ILSpy.TextView
 			var content = BuildHoverContent(model, segment);
 			if (content == null)
 				return;
-			lastTooltipSegment = segment;
 			if (content.Value.IsRich)
 				OpenRichPopup(content.Value.Control);
 			else
@@ -904,7 +902,6 @@ namespace ILSpy.TextView
 
 		void CloseTooltip()
 		{
-			lastTooltipSegment = null;
 			ToolTip.SetIsOpen(Editor.TextArea.TextView, false);
 		}
 
@@ -916,7 +913,6 @@ namespace ILSpy.TextView
 
 		void OnRichPopupClosed(object? sender, EventArgs e)
 		{
-			lastTooltipSegment = null;
 			distanceToPopupLimit = double.PositiveInfinity;
 		}