setup.py: remove namespace_packages declaration

Looks like the Setuptools people are moving to make this an error. I don't
think it was doing anything anyway.

https://bitbucket.org/pypa/setuptools/issue/12/update-or-remove-warning-when-namespace

setup.py

@@ -64,7 +64,6 @@ setup(name='beets',
           'beetsplug.web',
           'beetsplug.lastgenre',
       ],
-      namespace_packages=['beetsplug'],
       entry_points={
           'console_scripts': [
               'beet = beets.ui:main',