Consider deprecating LinkSettings accessor in Link

[sappenin]

The Link abstraction requires a LinkSettings, but it doesn't appear to be used in AbstractLink nor in IlpOverHttpLink. This is a good indicator that maybe we can remove this class, or else require this in the Constructor of each Link.

[sappenin]

Note for posterity: AccountSettings can have a map of custom-settings, but these are not necessary in a Link. Instead, each particular Link should require whatever it requires, and the LinkFactory is responsible for constructing the Link properly.