The auto-generated .c files could be removed

The git repository contains many .c files that could be generated by cython during the build process. 

I did the work for one of them. If you agree with the approach I'll do it for the others .c files.

Everything compiles fine with `make`. I also tested with this script:
```bash
#!/bin/bash
set -e

source /etc/bash_completion.d/virtualenvwrapper

virtualenv_name="sklearn_cythonize"

! mkvirtualenv base_with_numpy_scipy_cython
pip install numpy 
pip install scipy
pip install cython
! deactivate
! cpvirtualenv base_with_numpy_scipy_cython $virtualenv_name

# Test installing with pip
pip install -U -e git+https://github.com/NicolasTr/scikit-learn.git@cythonize#egg=scikit-learn-0.14 
python -c "import sklearn.tree;"

# Test building inplace from the top of the repository
cd $(virtualenvwrapper_derive_workon_home)/$virtualenv_name/src/scikit-learn
rm sklearn/tree/_tree.so
python setup.py build_ext --inplace
python -c "import sklearn.tree;"

# Test building inplace from the pyx file directory
cd sklearn/tree
rm _tree.so
python setup.py build_ext --inplace
python -c "import sklearn.tree;"
```