windows: remove fixup for contextvars

The extension no longer has a project file in 3.7.3. I believe I
contributed this patch upstream, heh.

Author: indygreg

cpython-windows/build.py

@@ -42,10 +42,6 @@
         'allow_missing_preprocessor': True,
     },
      '_bz2': {},
-    '_contextvars': {
-        # _contextvars.c is included in pythoncore for some reason.
-        'allow_missing_preprocessor': True,
-    },
     '_ctypes': {},
     '_decimal': {},
     '_elementtree': {},