Fix TextureAtlas remove-emitter leak on dispose

Dispose _onRemoveTextureAtlasCanvas in TextureAtlas.dispose() alongside
_onAddTextureAtlasCanvas to avoid leaking listeners when the atlas is torn down.

Fixes #5917

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

addons/addon-webgl/src/TextureAtlas.ts

@@ -109,6 +109,7 @@ export class TextureAtlas implements ITextureAtlas {
       page.canvas.remove();
     }
     this._onAddTextureAtlasCanvas.dispose();
+    this._onRemoveTextureAtlasCanvas.dispose();
   }
 
   public warmUp(): void {