Remove unused Image _expand()

[radarhere]

I suggest removing this unused function

Pillow/src/PIL/Image.py

Lines 1339 to 1343 in d42e537

	def _expand(self, xmargin: int, ymargin: int | None = None) -> Image:
	if ymargin is None:
	ymargin = xmargin
	self.load()
	return self._new(self.im.expand(xmargin, ymargin))

The C expand method is still used elsewhere.