Build system: remove toplevel target `fast`
Build system: remove toplevel target `fast`

Remove the recently introduced `fast` synonym for `fasttest`, because in
the subdirectories `make fast` already means `make all FAST=YES`.

[skip ci]

wxwxwwxxx/ghc,oldmanmike/ghc,shlevy/ghc,elieux/ghc,ml9951/ghc,snoyberg/ghc,sgillespie/ghc,ezyang/ghc,oldmanmike/ghc,ml9951/ghc,GaloisInc/halvm-ghc,gridaphobe/ghc,siddhanathan/ghc,gridaphobe/ghc,mettekou/ghc,tjakway/ghcjvm,nkaretnikov/ghc,shlevy/ghc,AlexanderPankiv/ghc,nushio3/ghc,siddhanathan/ghc,oldmanmike/ghc,mcschroeder/ghc,tjakway/ghcjvm,urbanslug/ghc,siddhanathan/ghc,AlexanderPankiv/ghc,nkaretnikov/ghc,sgillespie/ghc,nushio3/ghc,tjakway/ghcjvm,tjakway/ghcjvm,shlevy/ghc,mcschroeder/ghc,vikraman/ghc,oldmanmike/ghc,vTurbine/ghc,ml9951/ghc,gridaphobe/ghc,sdiehl/ghc,AlexanderPankiv/ghc,ezyang/ghc,sdiehl/ghc,siddhanathan/ghc,ezyang/ghc,vTurbine/ghc,wxwxwwxxx/ghc,ghc-android/ghc,da-x/ghc,nushio3/ghc,da-x/ghc,wxwxwwxxx/ghc,da-x/ghc,vTurbine/ghc,ml9951/ghc,vikraman/ghc,oldmanmike/ghc,urbanslug/ghc,oldmanmike/ghc,olsner/ghc,gridaphobe/ghc,TomMD/ghc,sdiehl/ghc,anton-dessiatov/ghc,AlexanderPankiv/ghc,wxwxwwxxx/ghc,anton-dessiatov/ghc,TomMD/ghc,mcschroeder/ghc,da-x/ghc,nushio3/ghc,ezyang/ghc,anton-dessiatov/ghc,vikraman/ghc,snoyberg/ghc,mfine/ghc,ghc-android/ghc,mettekou/ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,snoyberg/ghc,acowley/ghc,siddhanathan/ghc,urbanslug/ghc,gridaphobe/ghc,mfine/ghc,vikraman/ghc,gridaphobe/ghc,mcschroeder/ghc,ezyang/ghc,sgillespie/ghc,TomMD/ghc,snoyberg/ghc,wxwxwwxxx/ghc,TomMD/ghc,gridaphobe/ghc,sdiehl/ghc,vTurbine/ghc,da-x/ghc,mettekou/ghc,elieux/ghc,olsner/ghc,TomMD/ghc,nkaretnikov/ghc,GaloisInc/halvm-ghc,acowley/ghc,ghc-android/ghc,da-x/ghc,mcschroeder/ghc,snoyberg/ghc,mcschroeder/ghc,olsner/ghc,anton-dessiatov/ghc,mfine/ghc,sdiehl/ghc,vTurbine/ghc,snoyberg/ghc,sgillespie/ghc,vTurbine/ghc,siddhanathan/ghc,elieux/ghc,ezyang/ghc,mettekou/ghc,mcschroeder/ghc,mettekou/ghc,anton-dessiatov/ghc,mfine/ghc,nkaretnikov/ghc,anton-dessiatov/ghc,AlexanderPankiv/ghc,mfine/ghc,elieux/ghc,oldmanmike/ghc,vTurbine/ghc,vikraman/ghc,urbanslug/ghc,shlevy/ghc,olsner/ghc,urbanslug/ghc,nkaretnikov/ghc,sdiehl/ghc,urbanslug/ghc,nkaretnikov/ghc,tjakway/ghcjvm,nkaretnikov/ghc,olsner/ghc,snoyberg/ghc,mfine/ghc,sdiehl/ghc,vikraman/ghc,mettekou/ghc,nushio3/ghc,ml9951/ghc,AlexanderPankiv/ghc,nushio3/ghc,acowley/ghc,anton-dessiatov/ghc,sgillespie/ghc,tjakway/ghcjvm,TomMD/ghc,AlexanderPankiv/ghc,elieux/ghc,ml9951/ghc,TomMD/ghc,GaloisInc/halvm-ghc,mfine/ghc,sgillespie/ghc,acowley/ghc,siddhanathan/ghc,elieux/ghc,ghc-android/ghc,nushio3/ghc,ezyang/ghc,ml9951/ghc,elieux/ghc,shlevy/ghc,vikraman/ghc,mettekou/ghc,olsner/ghc,olsner/ghc,tjakway/ghcjvm,acowley/ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,sgillespie/ghc,ghc-android/ghc,wxwxwwxxx/ghc,shlevy/ghc,ghc-android/ghc,wxwxwwxxx/ghc,acowley/ghc,acowley/ghc,ml9951/ghc,shlevy/ghc,urbanslug/ghc,ghc-android/ghc,da-x/ghc