Need to review subband assignments -- needed?

[terrillmoore]

With the older LMIC, it was a great idea to use the subband restriction prior to joining, because the search algorithm was really slow (and could take arbitrarily long to reach a given subband). The algorithm is improved as of v4; so we don't need to restrict subbands.

Furthermore, the library interprets TTN US configuration "default" as requesting FSB2 (8~15), and prevents use of any other FSB. It would be better if "default" meant the same thing for all networks (as there's otherwise no way to get the default behavior of scanning all channels).