This removes the need to provide an undocumented launch argument to
enable custom RTMP service configurations to use multitrack encoding
and also provide a custom configuration.
While the Metal renderer is displayed as "Metal (Experimental)", the
"Experimental" part doesn't need to be stored to the config. Otherwise,
anyone having selected the Metal renderer will have it get unselected
once the "Experimental" part is removed, and presumably the people who
have it selected while it's experimental would also be the people who
would want it once it's stable.
