Remove misplaced/superfluous compatibility argument.
Remove misplaced/superfluous compatibility argument.

Obviously, this sentence is off-topic, it is neither about delegates nor
callable. It may support the _Why doesn't middleware use `__invoke`?_ reasons,
but if we rewrite and move it there, it would be superfluous: that section
already contains the compatibility argument:

> #### Why doesn't middleware use `__invoke`?
> Doing so would conflict with existing middleware that implements the
> double-pass approach and may want to implement the middleware interface.

Therefore, I suggest to simply remove it.

http-interop/http-middleware