Do not set compiler to mingw32 on Windows (#5)

With this patch, msvc is used, which is the default for building
Python extensions on Windows.
Currently, one must force usage of msvc when building bdist_wheel and
installing with something like:

%PYTHON% -m pip install . -vv --global-option build --global-option
--compiler=msvc --global-option build --global-option --cythonize

This kind of command will no longer work in the next major release of
pip: pypa/pip#8368

setuptools_cythonize.py

@@ -55,10 +55,6 @@ def initialize_options(self):
     def finalize_options(self):
         build.finalize_options(self)
 
-        if not self.compiler and platform.system() == 'Windows':
-            log.warn("No compiler provided, set it to mingw32 as default")
-            self.compiler = 'mingw32'
-
 
 class CythonizeBuildPy(build_py):