qapi: Bury some unused code in class Indentation

.__int__() has never been used.  Drop it.

.decrease() raises ArithmeticError when asked to decrease indentation
level below zero.  Nothing catches it.  It's a programming error.
Dumb down to assert.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210908045428.2689093-4-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

scripts/qapi/common.py

@@ -132,9 +132,6 @@ class Indentation:
     def __init__(self, initial: int = 0) -> None:
         self._level = initial
 
-    def __int__(self) -> int:
-        return self._level
-
     def __repr__(self) -> str:
         return "{}({:d})".format(type(self).__name__, self._level)
 
@@ -148,9 +145,7 @@ class Indentation:
 
     def decrease(self, amount: int = 4) -> None:
         """Decrease the indentation level by ``amount``, default 4."""
-        if self._level < amount:
-            raise ArithmeticError(
-                f"Can't remove {amount:d} spaces from {self!r}")
+        assert amount <= self._level
         self._level -= amount