Switched to saving 1-bit PDFs with DCTDecode

[radarhere]

Resolves #1775 - the remaining part of that issue is a request for compression when saving 1-bit PDFs.

The issue reports that

from PIL import Image
img = Image.new("1", (1400, 2000))
img.save("out.pdf")

generates a 5mb file.

Changing the filter to "DCTDecode", it becomes 34kb instead.

I've modified a test to ensure that saving hopper("1") as a PDF, that currently creates a 34kb file, stays below 15kb.

[radarhere]

This code is now unused, so I have removed it.