m4: readline: Drop extra_LIBS machinery

m4: readline: Drop extra_LIBS machinery

The first implementation of this logic was introduced with
commit 2ec759fc58fe all the way back in 2007; looking at the
build logs from our CI environment, however, it's apparent
that none of the platforms we currently target are actually
using it, so we can assume whatever issue it was working
around has been fixed at some point in the last 12 years.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>