opam: don't use the shell for no good reason

opam

@@ -8,7 +8,7 @@ license: "BSD"
 dev-repo: "https://gitlab.mpi-sws.org/FP/iris-coq.git"
 build: [make "-j%{jobs}%"]
 install: [make "install"]
-remove: ["sh" "-c" "rm -rf '%{lib}%/coq/user-contrib/iris'"]
+remove: ["rm" "-rf" "%{lib}%/coq/user-contrib/iris"]
 depends: [
   "coq" { >= "8.6.1" & < "8.8~" }
   "coq-mathcomp-ssreflect" { (>= "1.6.1" & < "1.7~") | (= "dev") }