Transport: Remove parsing of port ranges

UnicastZenDiscovery allowed you to specify port ranges for hosts to connect to.
However only the first port was used at all, when specifying something like
`host[9300-9400]`. This commit only accepts `host:port` pairs and thus makes
the configuration reflect the execution as even in the old setup only a single
host:port pair was pinged at all, as this was very expensive.

Note there is a coming PR for 1.x, which deprecates this setting and emits a warning instead of removing it.
And the related issue here: https://github.com/elasticsearch/elasticsearch/issues/7090