MAINT remove residual sparsefuncs*.so when compiling

The sparsefuncs -> sparsefuncs_fast rename in bbb8f1d has caused a lot of errors being reported (`ImportError: cannot import name 'inplace_column_scale'` or similar) due to dirty working copies. This attempts to avoid that by removing the old file explicitly. Ideally this deletion would only remain in the codebase for a short while, while <=0.14 users pull in the file rename.

The current solution may be too simple:
* many users may be using `python setup.py build --inplace` directly
* I do not think the current solution suffices in Windows