Remove mutableGrid() from GridTrackSizing Algorithm

https://bugs.webkit.org/show_bug.cgi?id=255738

Reviewed by Tim Nguyen.

mutableGrid() does not appear to be used anymore in the code base.

* Source/WebCore/rendering/GridTrackSizingAlgorithm.h:

Canonical link: https://commits.webkit.org/263207@main

Source/WebCore/rendering/GridTrackSizingAlgorithm.h

@@ -119,9 +119,6 @@ class GridTrackSizingAlgorithm final {
 
     // Required by RenderGrid. Try to minimize the exposed surface.
     const Grid& grid() const { return m_grid; }
-    // FIXME (jfernandez): We should remove any public getter for this attribute
-    // and encapsulate any access in the algorithm class.
-    Grid& mutableGrid() const { return m_grid; }
 
     const RenderGrid* renderGrid() const { return m_renderGrid; };