Remove all slirp4netns-specific setup and helper functions while
preserving the RootlessRLK port mapping functions that are still
used by pasta and bridge networking.
Rename networking_slirp4netns.go to networking_rootlessport.go.
Convert the slirp4netns compose test to use pasta instead.
Remove rootlessSlirpSyncR/W fields from container struct and
getSlirp4netnsIP stub from FreeBSD networking.
Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com>