use new default poster, remove black posters from icon cache

oml/media/epub.py

@@ -9,7 +9,6 @@ from io import BytesIO
 import re
 from urllib.parse import unquote
 
-from PIL import Image
 import stdnum.isbn
 from ox import strip_tags, decode_html
 
@@ -82,12 +81,6 @@ def cover(path):
                         img = os.path.normpath(os.path.join(os.path.dirname(filename), img))
                         if img in files:
                             return use(img)
-    # fallback return black cover
-    img = Image.new('RGB', (80, 128))
-    o = BytesIO()
-    img.save(o, format='jpeg')
-    data = o.getvalue()
-    o.close()
     return data
 
 def info(epub):