Remove mode and size checks from C transpose methods

[radarhere]

Given that _transpose sets imOut from imIn properties

Pillow/src/_imaging.c

Line 2162 in 11de4b9

imOut = ImagingNewDirty(imIn->mode, imIn->xsize, imIn->ysize);

I don't think that subsequent functions need to check the mode and size.

#9785 is an alternative that goes in the opposite direction, checking the return values and cleaning up on failure.

[akx]

I don't quite like this, because the loops just below assume that these preconditions hold.

[radarhere]

I'm aware there's a difference of opinion here. We'll see what another reviewer thinks.

[akx]

Also, per #9785 (comment) functions like ImagingTranspose and ImagingHorizontalBoxBlur are used by other Pillow functions, not only the Python interface _transpose...