numa: remove deprecated -mem-path fallback to anonymous RAM

it has been deprecated since 4.0 by commit
 cb79224b (deprecate -mem-path fallback to anonymous RAM)
Deprecation period ran out and it's time to remove it
so it won't get in a way of switching to using hostmem
backend for RAM.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200219160953.13771-2-imammedo@redhat.com>