Remove shebang from filesize.py

In #141 shebangs for `number.py` and `time.py` were removed, but
`filesystem.py` maintained it.

Signed-off-by: Mikel Olasagasti Uranga <mikel@olasagasti.info>

src/humanize/filesize.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-
 """Bits and bytes related humanization."""
 from __future__ import annotations