qapi/gen: Drop support for QAPIGen without a file name

The previous commit removed the only user of QAPIGen(None).  Tighten
the type hint.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210201193747.2169670-15-jsnow@redhat.com>

scripts/qapi/gen.py

@@ -40,7 +40,7 @@
 
 
 class QAPIGen:
-    def __init__(self, fname: Optional[str]):
+    def __init__(self, fname: str):
         self.fname = fname
         self._preamble = ''
         self._body = ''
@@ -125,7 +125,7 @@ def build_params(arg_type: Optional[QAPISchemaObjectType],
 
 
 class QAPIGenCCode(QAPIGen):
-    def __init__(self, fname: Optional[str]):
+    def __init__(self, fname: str):
         super().__init__(fname)
         self._start_if: Optional[Tuple[List[str], str, str]] = None