Deprecate -fwarn-hi-shadowing, an unimplemented/unused warning

Hello!

This is my first contribution to GHC, so I thought I would pick something simple. Issue #10913 (closed) suggests deprecating and eventually removing the -fwarn-hi-shadowing flag because it was not implemented correctly or used widely.

I have deprecated it in DynFlags.hs and removed it from the user guide, but I didn't add any unit tests for this case because this change seemed too simple to require a separate unit test that would only have to be removed when we remove this flag.