bootstrap: Don't allow rustfmt to fail on dist.

When running `x.py dist`, rustfmt was being allowed to fail when
missing-tools is true. This isn't much of an issue in practice
since other CI jobs will fail if rustfmt fails. This code was just
leftovers from when rustfmt was tracked in toolstate, and this removes
it to make it clear that it no longer works that way.

src/bootstrap/dist.rs

@@ -1226,17 +1226,10 @@ impl Step for Rustfmt {
 
         let rustfmt = builder
             .ensure(tool::Rustfmt { compiler, target, extra_features: Vec::new() })
-            .or_else(|| {
-                missing_tool("Rustfmt", builder.build.config.missing_tools);
-                None
-            })?;
+            .expect("rustfmt expected to build - essential tool");
         let cargofmt = builder
             .ensure(tool::Cargofmt { compiler, target, extra_features: Vec::new() })
-            .or_else(|| {
-                missing_tool("Cargofmt", builder.build.config.missing_tools);
-                None
-            })?;
-
+            .expect("cargo fmt expected to build - essential tool");
         let mut tarball = Tarball::new(builder, "rustfmt", &target.triple);
         tarball.set_overlay(OverlayKind::Rustfmt);
         tarball.is_preview(true);