Simplify MythCoreContext::GetMasterHostPrefix(). It's a waste of time to get the IP/port from the socket when we can be using the values from the settings instead, if we're not already connected to the master we will soon be connecting anyway to conduct the file transfer.
(cherry picked from commit 22b48f435a97b6b973a2e0dea9e89548350bc01e)

Refs #11937