Remove hardcoded image_shape from config.py

I previously pushed some updates that lets df3d determine the input
image size by just looking at the first image file, but I left the
default image size in config until being sure this worked. We've now run
df3d a lot on input images that have different sizes, and it works.
Running on different input sizes requires the user to _either_ update
the df3d.config['image_shape'] value for each dataset, or delete that
value which allows it to be automatically inferred. So let's just delete
the default value so that it can always automatically be inferred.

Author: jasper-tms

df3d/config.py

@@ -15,7 +15,6 @@
 config_fly = {
     "name": "fly",
     "num_cameras": 7,
-    "image_shape": [960, 480],
     "heatmap_shape": [64, 128],
     "left_cameras": [0, 1, 2],
     "right_cameras": [6, 5, 4],

df3d/core.py

@@ -96,7 +96,7 @@ def __init__(
                                  f" config.py image shape {self.image_shape}")
             self.image_shape = config["image_shape"] = image0_shape
         if not hasattr(self, "image_shape"):
-            raise ValueError("Image shape not specified in config and could"
+            raise ValueError("Image shape not specified in df3d.config and could"
                              f" not be read from {image0_path}")
 
         self.db = PoseDB(self.output_folder)