speed up x clean

Since `x clean` runs with bootstrap, we can speed up
this process by avoiding the cleaning of bootstrap artifacts,
as they are not necessarily needed to be cleaned.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/build_steps/clean.rs

@@ -139,7 +139,6 @@ fn clean_specific_stage(build: &Build, stage: u32) {
 fn clean_default(build: &Build) {
  rm_rf(&build.out.join("tmp"));
  rm_rf(&build.out.join("dist"));
- rm_rf(&build.out.join("bootstrap"));
  rm_rf(&build.out.join("rustfmt.stamp"));
 
  for host in &build.hosts {