Remove implicit-optional assignment in `__init__` that cannot ever be true

IfClauseNode is only ever initialized in such a way that this attribute
is immediately set to something valid. And attempting to access its
value when the value is None would be a pretty broken error anyway. The
assignment served no purpose, but did perform a frivolous runtime op in
addition to angering mypy's checks for implicit None.

mesonbuild/mparser.py

@@ -603,7 +603,6 @@ class IfClauseNode(BaseNode):
         super().__init__(linenode.lineno, linenode.colno, linenode.filename)
         self.ifs = []
         self.elseblock = EmptyNode(linenode.lineno, linenode.colno, linenode.filename)
-        self.endif = None
 
 @dataclass(unsafe_hash=True)
 class TestCaseClauseNode(BaseNode):