freemem: remove call to libxl_wait_for_free_memory

freemem: remove call to libxl_wait_for_free_memory

Now that libxl_wait_for_memory_target is capable of waiting until dom0
reaches its target, we can remove the other wait function call:
libxl_wait_for_free_memory. No need to wait twice. Once dom0 has met its
target, simply loop again and recalculate free_memkb.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Mike Latimer <mlatimer@suse.com>
Tested-by: Mike Latimer <mlatimer@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>