Completely remove gif support

Author: deshipu

stage.py

@@ -269,9 +269,7 @@ def from_bmp16(cls, filename):
     @classmethod
     def from_image(cls, filename):
         """Read the bank from an image file."""
-        if filename.lower().endswith(".gif"):
-            image = GIF16(filename)
-        elif filename.lower().endswith(".bmp"):
+        if filename.lower().endswith(".bmp"):
             image = BMP16(filename)
         elif filename.lower().endswith(".png"):
             image = PNG16(filename)