MAINTAINERS: Radim is no longer available as kvm-unit-tests maintainer

Radim's mail address @redhat.com is not valid anymore, so we should
remove this line from the MAINTAINERS file.

Thanks for all your work on kvm-unit-tests during the past years, Radim!

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191206131534.18509-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>