Software: Update Rust to 1.24.1

Update my Rust toolchain installation from version 1.22.1 to version
1.24.1 to escape a jobserver bug:
<rust-lang/rust#46981>.

Version 1.24.1 comes with rustfmt, so this patch uninstalls the Nix
package for rustfmt.

software/devel/languages/programming/rust/default.nix

@@ -6,10 +6,8 @@
   ];
   sw = p: with p; ([
     (rustChannelOf {
-      # The last version of 2017, as of 2017-12-30.
-      channel = "1.22.1";
+      channel = "1.24.1";
     }).rust
-    rustfmt
     rustracer
   ]);
 }