Fix testing good P mode BMP images

[radarhere]

Looking at the following code

Pillow/Tests/test_bmp_reference.py

Lines 105 to 107 in b0a5bc2

	im = im.convert("RGBA")
	compare = im.convert("RGBA")
	assert_image_similar(im, compare, 5)

it is readily apparent that it should read compare = compare.convert("RGBA").

Changing this does cause a file comparison to fail

Pillow/Tests/test_bmp_reference.py

Line 75 in b0a5bc2

"pal8nonsquare.bmp": "pal8nonsquare-v.png",

However, the test images have both pal8nonsquare-e.png and pal8nonsquare-v.png. Changing the test to compare against pal8nonsquare-e.png causes it to pass. https://github.com/jsummers/bmpsuite/blob/master/html/bmpsuite.html describes the difference as

Image editors can be expected to
leave the image “squashed”; image viewers should
consider stretching it to its correct proportions.

While that's not the clearest distinction for Pillow, this PR is just editing our test suite. Since we favour backwards compatibility, I'm going to choose pal8nonsquare-e.png, saying that Pillow is an 'image editor', since we currently leave the image squashed.

While I'm here, I'm also going to remove a number of comparison BMP images that we don't actually use, and the BMP HTML documentation.