dockerd-rootless.sh: remove confusing code comment
`--userland-proxy-path` is automatically set by dockerd:
#
# Usage: dockerd-rootless.sh --experimental [DOCKERD_OPTIONS]
# Currently, specifying --experimental is mandatory.
-# Also, to expose ports, you need to specify
-# --userland-proxy-path=/path/to/rootlesskit-docker-proxy
#
# External dependencies:
# * newuidmap and newgidmap needs to be installed.
