Get rid of guess_cover()

It was returning a bool rather than a path before and no one seemed to
have noticed, so it cant be needed.

See #1678 (opf2.py's OPF.guess_cover doesn't return the path)

src/calibre/ebooks/metadata/opf2.py

@@ -1158,26 +1158,6 @@ def primary_writing_mode(self):
  for m in self.XPath('//*[local-name()="meta" and @name="primary-writing-mode" and @content]')(self.root):
  return m.get('content')
 
- def guess_cover(self):
- '''
- Try to guess a cover. Needed for some old/badly formed OPF files.
- '''
- if self.base_dir and os.path.exists(self.base_dir):
- for item in self.identifier_path(self.metadata):
- scheme = None
- for key in item.attrib.keys():
- if key.endswith('scheme'):
- scheme = item.get(key)
- break
- if scheme is None:
- continue
- if item.text:
- prefix = item.text.replace('-', '')
- for suffix in ['.jpg', '.jpeg', '.gif', '.png', '.bmp']:
- cpath = os.access(os.path.join(self.base_dir, prefix+suffix), os.R_OK)
- if os.access(os.path.join(self.base_dir, prefix+suffix), os.R_OK):
- return cpath
-
  @property
  def epub3_raster_cover(self):
  for item in self.itermanifest():
@@ -1239,11 +1219,6 @@ def cover(self):
  for item in self.guide:
  if item.type and item.type.lower() == t:
  return item.path
- try:
- if self.try_to_guess_cover:
- return self.guess_cover()
- except:
- pass
 
  @cover.setter
  def cover(self, path):