`extra-library-flavours` and `extra-dynamic-library-flavours` should be removed

[hasufell]

Unless I missed something, these options have no documentation and no purpose other than facilitating a hack for GHCs rts package: https://github.com/ghc/ghc/blob/3d3058895c0dac15c9ee4f88c15cacaca988b1d5/rts/rts.cabal#L174-L206