build: cleanup unreferenced CMake variables
build: cleanup unreferenced CMake variables

`SWIFT_CMARK_LIBRARY_DIR` is not referenced in the build, remove the
variable.

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