Cleanup param "workers" in WSGIMiddleware (#1146)

Param "workers" in WSGIMiddleware.__init__ has not been used
since 0.6.3, which is changed in GH-164, commit 96c51c.

Co-authored-by: Jamie Hewland <jhewland@gmail.com>

starlette/middleware/wsgi.py

@@ -54,7 +54,7 @@ def build_environ(scope: Scope, body: bytes) -> dict:
 
 
 class WSGIMiddleware:
-    def __init__(self, app: typing.Callable, workers: int = 10) -> None:
+    def __init__(self, app: typing.Callable) -> None:
         self.app = app
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None: