Remove shebang from graph.py

[mtelka]

The shebang is useless in the graph.py file:

$ chmod +x dulwich/graph.py
$ PYTHONPATH=. dulwich/graph.py
Traceback (most recent call last):
  File "/tmp/dulwich/dulwich/graph.py", line 25, in <module>
    from .lru_cache import LRUCache
ImportError: attempted relative import with no known parent package
$

So let's remove it.

See also #1230.

[mtelka]

The issue was introduced via #1198.