package/nss-mdns: do not overwrite user's nsswitch.conf

Instead of over-writing the nsswitch.conf file (which may already contain
user's specific configuration if it comes from a custom skeleton), just
replace the 'hosts' line.

Remove now-useless custom nsswitch.conf file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>