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

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

Start by reviewing the linked GHC rts/rts.cabal section at lines 174–206, then search the Cabal repository for definitions and uses of both option names. Done means the options are removed along with references that only support the GHC RTS hack, with no remaining documented or functional purpose.

Written by the indexing model from the issue text.