rts: retainer: Remove obsolete debug code
rts: retainer: Remove obsolete debug code

Commit dbef766ce7 ("Profiling cleanup.") made this debug code obsolete by
removing the 'cost' function without a replacement. As best I can tell the
retainer profiler used to do some heap census too and this debug code was
mainly concerned with that.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc