Provide default if .pop(...) errors (#655)

As `.pop(...)` already catches the `KeyError` if a default value is
supplied, use that to simplify the code a bit.

Author: jakirkham

zarr/n5.py

@@ -311,11 +311,8 @@ def group_metadata_to_n5(group_metadata):
 
 def group_metadata_to_zarr(group_metadata):
     '''Convert group metadata from N5 to zarr format.'''
-    try:
-        group_metadata.pop('n5')
-    except KeyError:
-        # This only exists at the top level
-        pass
+    # This only exists at the top level
+    group_metadata.pop('n5', None)
     group_metadata['zarr_format'] = ZARR_FORMAT
     return group_metadata