MAINTAINERS: remove drivers/staging/hv/

This directory was removed by commit 89ae7d70 (hv: storvsc: Move the
storage driver out of the staging area).

Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>