GIF: Remove gif_resize handler

[dfdan]

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Description

create_gif_from_frames / resize_gif were throwing errors with current versions of Pillow. On closer investigation, these had no effect even when working there's so I've removed them.

The functions:

• Treated alll GIFs as a sequence.
• Only gave resize special treatment - all other IIIF operations reduce the GIF to its opening frame (- which in most cases is preferable, since -)
• IIIF doesn't play well with animated GIFs - a primary use case is tiling large images and the tiles are not synronizable.

(The original animated GIF is still downloadable, unmodified by the IIIF API as with other files)

Closes #91

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