bpo-34687: Update asyncio doc for ProactorEventLoop (GH-9623)

Since ProactorEventLoop is now the default in 3.8, remove examples
using it explicitly on Windows.





https://bugs.python.org/issue34687

Doc/library/asyncio-protocol.rst

@@ -1030,10 +1030,6 @@ The subprocess is created by th :meth:`loop.subprocess_exec` method::
         data = bytes(protocol.output)
         return data.decode('ascii').rstrip()
 
-    if sys.platform == "win32":
-        asyncio.set_event_loop_policy(
-            asyncio.WindowsProactorEventLoopPolicy())
-
     date = asyncio.run(get_date())
     print(f"Current date: {date}")