remove HAVE_NEWUIDMAP and NEWUIDMAP

Always build lxc-usernsexec.  Else we require having uidmap
installed on the build host for no good reason.  And we never
actually used the NEWUIDMAP path we detected.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>