Clean up param "workers" in WSGIMiddleware

[laggardkernel]

Param workers in WSGIMiddleware.__init__ has not been used
since 0.6.3, which is changed in GH-164, commit 96c51c9.

We formerly used a ThreadPoolExecutor, which support setting worker numbers. Later it was changed to used the default executor.

# 0.5.2 2018-10-17 860fdf6c8b0eb88b9668153fdf2bf0352ec3152a

class WSGIMiddleware:
    def __init__(self, app: typing.Callable, workers: int = 10) -> None:
        self.app = app
        self.executor = ThreadPoolExecutor(max_workers=workers)

    async def __call__(self, receive: Receive, send: Send) -> None:
        ...
        wsgi = self.loop.run_in_executor(
            self.executor, self.wsgi, environ, self.start_response
        )


# 0.6.3 2018-10-31 96c51c959ae5592194f0b785a424e225cd4f4d3c

class WSGIMiddleware:
    ...
    async def __call__(self, receive: Receive, send: Send) -> None:
        # custom executor is dropped
        wsgi = self.loop.run_in_executor(
            None, self.wsgi, environ, self.start_response
        )

I don't think anyone is still using the "worker" param after more than 2 years. But out of caution, only made a DeprecationWarning first. Cause I'm not sure which is the correct time to obsolete this param in init, just gave a release number "1.0", which may need to be adjusted. The parameter was removed.

[laggardkernel]

I was just watching the code and finding how Starlette works, noticed there's a weird param "workers" not needed by any functions, and opened a pr.

I noticed a thumb down here. Sorry to submit such a trifling change. Maybe this pr is unnecessary cause the WSGIMiddleware.__init__ still works without the "workers" removed. Or maybe just better to notice anyone uses "workers" and tell them it had been dropped.

I'll handle the decision to the maintainers.

BTW, force pushed once to fix linting. The checks reported a coverage fail, not sure how it's related to my commit.

[JayH5]

Thanks for the PR. I think we can just remove this parameter since it hasn't actually done anything for a long time.

[laggardkernel]

Changed the fix to cleaning up.