stop advertising the running of tests to end users

I've realized that this was broken. (The tests were not installed and could be
only run for inplace-builds linked from somewhere else.)  Fixing this would be
somewhat hacky.  I think now that it was a bad idea in the first place to
advertise the running of unit tests to end users.

INSTALL.txt

@@ -27,11 +27,6 @@ The following software is highly recommended though not strictly required:
    that will in many cases speed up kwant several times and reduce the memory
    footprint.
 
-Suggested:
-
- - The `nose <http://nose.readthedocs.org/>`_ testing framework is required to
-   run the extensive tests included with kwant.
-
 Generic instructions
 ====================
 
@@ -51,12 +46,6 @@ subdirectory, e.g.::
     cd tutorial
     python 1-quantum_wire_revisited.py
 
-If "nose" is installed, you can also run the tests ::
-
-    python -c 'import kwant; kwant.test()'
-
-from some directory *other* than the kwant distribution's root directory.
-
 
 System-specific instructions
 ============================
@@ -262,5 +251,4 @@ extension).  You will need Cython 0.17.1 if you want to modify them.
 kwant includes extensive unit tests which are supposed to cover all of its
 functionality.  We use the `nose <http://nose.readthedocs.org/>`_ testing
 framework.  To run the tests, execute the command ``nosetests`` from the root
-directory of the package after it has been built in place.  You can also
-``import kwant`` inside Python and run ``kwant.test()``.
+directory of the package after it has been built in place.

kwant/__init__.py

@@ -27,22 +27,3 @@ try:
 except ImportError:
     from .solvers.sparse import solve
     __all__.append('solve')
-
-def runtest():
-    """Run the tests included with kwant."""
-    import os.path
-    try:
-        import nose
-    except ImportError:
-        msg = 'No module named nose.\n(See INSTALL.txt in ' \
-            'the top-level directory of the kwant distribution.)'
-        raise ImportError(msg)
-    kwant_dir = os.path.dirname(__file__)
-    if os.path.basename(kwant_dir) == kwant_dir:
-        msg = 'Do not call kwant.test from within the kwant source ' \
-        'distribution directory.\nYou can run "nosetests -s" instead.'
-        raise RuntimeError(msg)
-    nose.run(defaultTest=kwant_dir)
-
-# This trick avoids recursive execution of tests by nosetests.
-test = runtest