Unused flag -ddump-shape

That flag was added as part of 00b530d5 without being used, and still isn't used, so we should get rid of it.

It is represented by Opt_D_dump_shape, defined here: https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/main/DynFlags.hs#L453

and is mentioned another time or two in that module (but nowhere else).