Remove default entries from composites configuration

Defining ExposureF and calexp disassembly behavior in the default
configuration did not gain us anything over the default "false".
It was there for historical purposes but it now confuses downstream
overrides.

python/lsst/daf/butler/configs/datastores/composites.yaml

@@ -8,6 +8,6 @@ default: false
 # Types can be StorageClass names or DatasetType names, with DatasetType
 # name always taking preference over StorageClass name.
 disassembled:
-  ExposureI: false
-  ExposureF: false
-  calexp: false
+  # This is a place holder to seed the mapping so that specific configs
+  # can override and allows this default to be used as documentation
+  __null__: false

python/lsst/daf/butler/core/composites.py

@@ -83,9 +83,18 @@ def __init__(self, config: Union[str, ButlerConfig, CompositesConfig], *,
         assert isinstance(config, CompositesConfig)
         self.config = config
 
+        # Pre-filter the disassembly lookup table to remove the
+        # placeholder __ key we added for documentation.
+        # It should be harmless but might confuse validation
+        # Retain the entry as a Config so change in place
+        disassemblyMap = self.config[DISASSEMBLY_KEY]
+        for k in set(disassemblyMap):
+            if k.startswith("__"):
+                del disassemblyMap[k]
+
         # Calculate the disassembly lookup table -- no need to process
         # the values
-        self._lut = processLookupConfigs(self.config[DISASSEMBLY_KEY], universe=universe)
+        self._lut = processLookupConfigs(disassemblyMap, universe=universe)
 
     def shouldBeDisassembled(self, entity: Union[DatasetRef, DatasetType, StorageClass]) -> bool:
         """Given some choices, indicate whether the entity should be

tests/test_composites.py

@@ -43,10 +43,8 @@ def testConfig(self):
         self.assertIn("default", c)
         # Check merging has worked
         rootKey = "disassembled"
-        self.assertIn(f".{rootKey}.calexp", c)
         self.assertIn(f".{rootKey}.dummyTrue", c)
         self.assertIn(f".{rootKey}.StructuredComposite", c)
-        self.assertIn(f".{rootKey}.ExposureF", c)
 
         # Check that all entries are booleans (this is meant to be enforced
         # internally)