Remove Feature module to match setuptools >=46.0.0

pypa/setuptools#65
pypa/setuptools#2017
openslide/openslide#287

Author: userkkw

setup.py

@@ -1,5 +1,5 @@
 import os
-from setuptools import setup, Extension, Feature
+from setuptools import setup, Extension
 
 # Load version string
 _verfile = os.path.join(os.path.dirname(__file__), 'openslide', '_version.py')
@@ -15,15 +15,9 @@
     packages=[
         'openslide',
     ],
-    features={
-        'performance': Feature(
-            'performance-enhancing extension module',
-            standard=True,
-            ext_modules=[
-                Extension('openslide._convert', ['openslide/_convert.c']),
-            ],
-        ),
-    },
+    ext_modules=[
+        Extension('openslide._convert', ['openslide/_convert.c']),
+    ],
     test_suite='tests',
     maintainer='OpenSlide project',
     maintainer_email='[email protected]',