Remove backwards compatibility to read files

without information about the input visits.

python/lsst/cell_coadds/_single_cell_coadd.py

@@ -85,8 +85,7 @@ def __init__(
         self._common = common
         # Remove any duplicate elements in the input, sorted them and make
         # them an immutable sequence.
-        # TODO: Remove the conditioning in DM-40563.
-        self._inputs = tuple(sorted(set(inputs))) if inputs else ()
+        self._inputs = tuple(sorted(set(inputs)))
         self._identifiers = identifiers
 
     @property
@@ -107,8 +106,7 @@ def psf_image(self) -> ImageF:
         return self._psf
 
     @property
-    # TODO: Remove the conditioning in DM-40563.
-    def inputs(self) -> tuple[ObservationIdentifiers, ...] | tuple[()]:
+    def inputs(self) -> tuple[ObservationIdentifiers, ...]:
         """Identifiers for the input images that contributed to this cell,
         sorted by their `packed` attribute.
         """