Simplify Hotspot logic by removing Connection: drop-down

[jlnr]

I don't find the "Connection" drop-down in the Hotspot page very intuitive. I thought it'd let me choose which Ethernet connection to share (I have two), but instead it lets me manage all my hotspot connections (SSID/password combinations from the user's POV).

What is the use case for this drop-down? Do people change their hotspot SSIDs often enough that they don't want to re-enter these details every time?

Suggested fix: Remove the dropdown, and silently replace the previous hotspot configuration when the user sets a new SSID or password.

Alternatively, add an explanation below the drop-down to indicate that it lets me manage a collection of credentials.

(I'm using the page layout from #176 right now, but I remember being just as confused on the master branch)