Remove napari-spatialdata PySide6 workaround once fixed upstream

[fjorka]

Four notebooks in notebooks/extras/ (instanseg_demo, viewer_demo, 06_QC_after_quant_demo, cellpose_demo) currently alias pyqtgraph.Qt.QtCore.pyqtSignal = qtpy.QtCore.Signal before importing napari_spatialdata.

Why: napari-spatialdata is unimportable under PySide6. See the upstream PR: scverse/napari-spatialdata#399.

When the fix is released upstream:

• Delete the alias from all four notebooks.
• Raise the napari-spatialdata floor in pyproject.toml to the first fixed release.