Remove mirrored_supervisor from erl_first_files

Module mirrored_supervisor in erl_first_files cause compilation errors on some systems due to rebar feature. Rebar does not depend on order of files in erl_first_files list but on order of files in list returned by filelib:fold_files/5. Since no one module of rabbit_common project depends on mirrored_supervisor we can remove it from erl_first_files list without any consequences.