Rm `ImageSource.__deepcopy__` workaround after FINUFFT Release.

[garrettwrong]

          I tested the `as_copy` behavior against the current HEAD on `finufft` (`a0db2d0b`), and we don't need the hack we're using if we use that version of finufft. So when we update our dependencies, we can remove the entire `__deepcopy__` override for `ImageSource`.

          Of course there may be other similar or unrelated hacks we need to do for other nfft backends that I haven't tested, so it may be worthwhile to make a historical note of it somewhere.

Originally posted by @vineetbansal in #902 (comment)

[garrettwrong]

Attempting to remove this in #1168