update opam file

opam

@@ -3,7 +3,7 @@ name: "coq-fri"
 maintainer: "Joseph Tassarotti"
 build: [make "-j%{jobs}%"]
 install: [make "install"]
-remove: [ "sh" "-c" "rm -rf '%{lib}%/coq/user-contrib/fri" ]
+remove: ["rm" "-rf" "'%{lib}%/coq/user-contrib/fri"]
 depends: [
   "coq" { >= "8.7" }
   "coq-iris"  { (= "branch.gen_proofmode.2018-03-04.7") | (= "dev") }