Magic Folder: default [magic_folder] section of tahoe.cfg is no longer needed

[daira]

Originally, the default tahoe.cfg created for a new node included a [drop_upload] section, later renamed to [magic_folder], because the user was expected to manually edit the local.directory entry.

This is no longer needed because the user is expected to always use tahoe magic-folder join, so it should be removed.

[dawuud]

excluded stats variable removed here: ​https://github.com/tahoe-lafs/tahoe-lafs/pull/210

[daira]

Please fix the test failure:

allmydata.test.test_runner.CreateNode.test_client
===============================================================================
[FAIL]
Traceback (most recent call last):

  File "/home/travis/build/tahoe-lafs/tahoe-lafs/src/allmydata/test/test_runner.py", line 330, in test_node
    self.do_create("node")

  File "/home/travis/build/tahoe-lafs/tahoe-lafs/src/allmydata/test/test_runner.py", line 269, in do_create
    self.failUnless(re.search(r"\n\[magic_folder\]\n#.*\n#enabled = false\n", content), content)

  File "/home/travis/build/tahoe-lafs/tahoe-lafs/support/lib/python2.7/site-packages/Twisted-15.5.0-py2.7-linux-x86_64.egg/twisted/trial/_synctest.py", line 400, in assertTrue
    super(_Assertions, self).assertTrue(condition, msg)

  File "/opt/python/2.7.9/lib/python2.7/unittest/case.py", line 422, in assertTrue
    raise self.failureException(msg)

(I think that failUnless just needs to be removed.)

[daira]

Merged into 2438.magic-folder-stable.5.