Remove unnecessary code in WmfHandler (#9411)

Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>

Author: radarhere

src/PIL/WmfImagePlugin.py

@@ -45,7 +45,6 @@ def register_handler(handler: ImageFile.StubHandler | None) -> None:
 
     class WmfHandler(ImageFile.StubHandler):
         def open(self, im: ImageFile.StubImageFile) -> None:
-            im._mode = "RGB"
             self.bbox = im.info["wmf_bbox"]
 
         def load(self, im: ImageFile.StubImageFile) -> Image.Image: