magit-read-org-clubhouse-branch-name returns the branch with the prefix
in it already, so we don't need to add it again here.
Change-Id: I0e753173bc366a8458ccd38a936ae078bbac79f8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1999
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>