buildman: Drop a TODO that is done

Buildman now uses worktrees when available, instead of doing a full clone.
This was done in this commit:

   76de29fc4f buildman: Use git worktrees instead of git clones when possible

Drop the TODO.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/README

@@ -1331,9 +1331,6 @@ scope for more though, e.g.:
 - 'hunting' for problems, perhaps by building a few boards for each arch, or
   checking commits for changed files and building only boards which use those
   files
-- using the same git repo for all threads instead of cloning it. Currently
-  it uses about 500MB per thread, so on a 64-thread machine this is 32GB for
-  the build.
 
 
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