libqemustub.a has been removed in commit ebedb37c ("Makefile: Remove
libqemustub.a"). Some remainders have been missed. Remove them now.

Message-Id: <20200804170055.2851-8-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>