This abstracts more of the help text into the ScanModule definition,
removing some more of the need for `zgrab2.AddCommand()`
https://github.com/zmap/zgrab2/pull/248
The previous patch allows the port to be specified in the
`ScanTarget{}`.
Since the port option in the Config may not be the port currently being
scanned, delete the `GetPort()` function provided by each module.
The `GetPort()` function is also not used. While we could just change
the meaning of this function, to mean "Return the port in the Config",
it is probably better to go ahead and just remove all references to it
as there are no users.
