install: Avoid a superfluous prompt to choose the only remote

The install command can search available remotes for a specified flatpak
when a remote wasn't specified. In case only one remote is configured,
or in case only one of the configured remotes matches the ref specified,
we currently prompt the user to confirm use of the remote anyway (unless
-y/--assumeyes was used). Skip this prompt even when -y/--assumeyes was
not used, since the remote to use will still effectively be confirmed
when the list of refs to be installed is presented for confirmation.

Fixes https://github.com/flatpak/flatpak/issues/4364