Removed setters for read-only properties

[radarhere]

Suggestion for python-pillow#7271, to fix the failing tests.

I've removed the setters for mode and size, restored some of the lines you removed, removed a duplicate @property and updated a BMP test and GifImagePlugin.

You have considered that the image might not be loaded yet, meaning that there is no self.im. I think you also need to consider seeking to another frame, but before, at which time that there will be a self.im but the size of the image should be the size of the tile that isn't loaded yet. As a result, I've also added a @property in ImageFile for size. I've also added one for mode.

With these changes, all tests now pass.

[Yay295]

I can see that six tests fail (in my local environment) if they aren't, but I don't understand why setting _mode and _size in these two locations is necessary:
https://github.com/radarhere/Pillow/blob/400b216630bc86f2b1fff386469e68b4c8d48fc2/src/PIL/Image.py#L508-L512
https://github.com/radarhere/Pillow/blob/400b216630bc86f2b1fff386469e68b4c8d48fc2/src/PIL/Image.py#L694-L699
Since im is being set at the same time, the mode and size properties should be getting their data from im, so it shouldn't be necessary to also set _mode and _size.

[radarhere]

It is because an ImagingCore instance with one dimension of zero is falsy.

>>> from PIL import Image
>>> "truthy" if Image.new("L", (0, 1)).im else "falsy"
'falsy'
>>> "truthy" if Image.new("L", (1, 0)).im else "falsy"
'falsy'
>>> "truthy" if Image.new("L", (1, 1)).im else "falsy"
'truthy'

This is producing results that you don't expect when running against self.im.mode if self.im else self._mode.

If you dislike this, then you can either change the self.im check to compare against None instead, or try and change how ImagingCore instances evaluate to booleans.