build-sys: remove useless extra*flags variables

Only EXTRA_LDFLAGS seems to be used during configure Xen checks.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180208162343.30809-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>