Remove check for /ImageMask

/ImageMask means the the image is a stencil mask for a grayscale or
color image. From issue #63 a user has a PDF apparently with only a
stencil mask and no other images. According to PDF spec you're supposed
to use masks to help draw other images rather than draw masks
explicitly, although drawing masks is not forbidden.

In the event that the image mask is higher resolution than the input
 image, the composite image should be drawn at the highest DPI to a
ccurately capture the effect of the mask. It looks like the right to
 do is take masks into account rather than ignore them (a holdover
from earlier, less robust versions), or don't treat it different.

ocrmypdf/pageinfo.py

@@ -201,9 +201,6 @@ def _find_page_images(page, pageinfo, contentsinfo):
         pdfimage = page['/Resources']['/XObject'][xobj]
         if pdfimage['/Subtype'] != '/Image':
             continue
-        if '/ImageMask' in pdfimage:
-            if pdfimage['/ImageMask']:
-                continue
         image = {}
         image['name'] = str(xobj)
         image['width'] = pdfimage['/Width']