racerRust: Use stable rust platform

pkgs/top-level/all-packages.nix

···
 
   premake = premake4;
 
-  racerRust = callPackage ../development/tools/rust/racer {
-    # racerRust still uses unstable features from the standard library
-    rustPlatform = rustUnstable;
-  };
+  racerRust = callPackage ../development/tools/rust/racer { };
 
   radare = callPackage ../development/tools/analysis/radare {
     inherit (gnome) vte;