build: update shell.nix (#8616)

substrate/shell.nix

@@ -5,14 +5,13 @@ let
       rev = "57c8084c7ef41366993909c20491e359bbb90f54";
     });
   nixpkgs = import <nixpkgs> { overlays = [ mozillaOverlay ]; };
-  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2020-10-23"; channel = "nightly"; }).rust.override {
+  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2021-03-01"; channel = "nightly"; }).rust.override {
     targets = [ "wasm32-unknown-unknown" ];
   });
 in
 with nixpkgs; pkgs.mkShell {
   buildInputs = [
     clang
-    cmake
     pkg-config
     rust-nightly
   ] ++ stdenv.lib.optionals stdenv.isDarwin [