Remove the number field from the PredNode union type.

Predicate numbers are available in the maps
isa.settings.predicate_number and isa.instp_number instead.

Like the name field, predicate numbers don't interact well with
unique_pred().

lib/cretonne/meta/cdsl/settings.py

@@ -23,7 +23,6 @@ class Setting(object):
     def __init__(self, doc):
         # type: (str) -> None
         self.name = None  # type: str  # Assigned later by `extract_names()`.
-        self.number = None  # type: int
         self.__doc__ = doc
         # Offset of byte in settings vector containing this setting.
         self.byte_offset = None  # type: int