build: remove remnants of libdispatch staging (NFC)
build: remove remnants of libdispatch staging (NFC)

libdispatch is required by Foundation.  The
`FOUNDATION_ENABLE_LIBDISPATCH` flag was removed a while ago from
Foundation itself.  Remove the handling for this option and simplify the
flag handling.

allevato/swift,gribozavr/swift,apple/swift,glessard/swift,gribozavr/swift,hooman/swift,ahoppen/swift,glessard/swift,stephentyrone/swift,JGiola/swift,benlangmuir/swift,atrick/swift,glessard/swift,parkera/swift,JGiola/swift,tkremenek/swift,CodaFi/swift,benlangmuir/swift,jmgc/swift,aschwaighofer/swift,apple/swift,nathawes/swift,stephentyrone/swift,jmgc/swift,xwu/swift,ahoppen/swift,apple/swift,jmgc/swift,allevato/swift,rudkx/swift,rudkx/swift,parkera/swift,stephentyrone/swift,jckarter/swift,hooman/swift,roambotics/swift,tkremenek/swift,JGiola/swift,allevato/swift,aschwaighofer/swift,allevato/swift,gribozavr/swift,rudkx/swift,jckarter/swift,parkera/swift,jmgc/swift,nathawes/swift,ahoppen/swift,rudkx/swift,airspeedswift/swift,JGiola/swift,allevato/swift,nathawes/swift,gregomni/swift,atrick/swift,gribozavr/swift,airspeedswift/swift,aschwaighofer/swift,airspeedswift/swift,airspeedswift/swift,gribozavr/swift,ahoppen/swift,xwu/swift,benlangmuir/swift,jckarter/swift,parkera/swift,tkremenek/swift,harlanhaskins/swift,gribozavr/swift,harlanhaskins/swift,gregomni/swift,roambotics/swift,aschwaighofer/swift,xwu/swift,tkremenek/swift,roambotics/swift,benlangmuir/swift,ahoppen/swift,hooman/swift,benlangmuir/swift,aschwaighofer/swift,glessard/swift,JGiola/swift,harlanhaskins/swift,stephentyrone/swift,roambotics/swift,benlangmuir/swift,CodaFi/swift,roambotics/swift,jckarter/swift,xwu/swift,glessard/swift,jmgc/swift,gregomni/swift,tkremenek/swift,allevato/swift,parkera/swift,aschwaighofer/swift,ahoppen/swift,atrick/swift,stephentyrone/swift,gregomni/swift,parkera/swift,gribozavr/swift,jmgc/swift,nathawes/swift,jmgc/swift,hooman/swift,harlanhaskins/swift,apple/swift,jckarter/swift,JGiola/swift,hooman/swift,aschwaighofer/swift,harlanhaskins/swift,parkera/swift,airspeedswift/swift,stephentyrone/swift,stephentyrone/swift,hooman/swift,xwu/swift,tkremenek/swift,harlanhaskins/swift,jckarter/swift,atrick/swift,rudkx/swift,CodaFi/swift,CodaFi/swift,apple/swift,tkremenek/swift,xwu/swift,gribozavr/swift,apple/swift,airspeedswift/swift,nathawes/swift,CodaFi/swift,hooman/swift,gregomni/swift,allevato/swift,parkera/swift,roambotics/swift,jckarter/swift,CodaFi/swift,nathawes/swift,nathawes/swift,CodaFi/swift,glessard/swift,airspeedswift/swift,xwu/swift,rudkx/swift,atrick/swift,atrick/swift,gregomni/swift,harlanhaskins/swift