Clean target for 3rdparty
Clean target for 3rdparty

(useful after opam switch)

hoheinzollern/FStar,YaccConstructor/FStar,YaccConstructor/FStar,MallabaevAzamat/FStar,msprotz/FStar,YaccConstructor/FStar,aa755/FStar,aa755/FStar,A-Manning/FStar,msprotz/FStar,aa755/FStar,hoheinzollern/FStar,A-Manning/FStar,FStarLang/FStar,msprotz/FStar,MallabaevAzamat/FStar,hoheinzollern/FStar,A-Manning/FStar,aa755/FStar,FStarLang/FStar,MallabaevAzamat/FStar,FStarLang/FStar,YaccConstructor/FStar,hoheinzollern/FStar,MallabaevAzamat/FStar,msprotz/FStar