remove stale references to /tutorial

This completes 9fdd41e8.

.gitignore

@@ -4,7 +4,6 @@
 *.so
 /kwant/*.c
 /kwant/*/*.c
-/tutorial
 /build
 /dist
 /doc/build

setup.py

@@ -250,8 +250,7 @@ def git_lsfiles():
 
 # Make the command "sdist" depend on "build".  This verifies that the
 # distribution in the current state actually builds.  It also makes sure that
-# the Cython-made C files and the tutorial will be included in the source
-# distribution and that they will be up-to-date.
+# the Cython-made C files will be up-to-date and included in the source.
 class sdist(sdist_orig):
     sub_commands = [('build', None)] + sdist_orig.sub_commands