Remove the now unused MSS.display class variable from GNU/Linux implementation

Finalisation of 59c2259.

Author: BoboTiG

CHANGES.rst

@@ -17,6 +17,7 @@ darwin.py
 linux.py
 --------
  - Added ``MSS.has_extension()``
+ - Removed ``MSS.display``
  - Renamed ``MSS.grab()`` to ``MSS._grab_impl()``
  - Renamed ``MSS.monitors`` to ``MSS._monitors_impl()``
 

mss/linux.py

@@ -184,10 +184,6 @@ class MSS(MSSBase):
 
     __slots__ = {"drawable", "root", "xlib", "xrandr"}
 
-    # Class attribute to store the display opened with XOpenDisplay().
-    # Instancied one time to prevent resource leak.
-    display = None
-
     # A dict to maintain *display* values created by multiple threads.
     _display_dict = {}  # type: Dict[threading.Thread, int]