remove --root-cmd from the cabal options that ghcjs-boot uses
remove --root-cmd from the cabal options that ghcjs-boot uses

The --root-cmd option has been dropped from Cabal 1.25 and it was only used
by ghcjs-boot to override any existing custom setting. Since we are now
pointing to our own cabal config file to start with a clean configuration,
it is not necessary anymore.

ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs