message: docs: remove unused release instruction
  Turns out since we use `git archive`, there's no need for `git clean`
  which was pretty disruptive.
diff: @@ -61,12 +61,6 @@ Now let's craft a source release
  # Let's create a git tag
  git tag -f ${VERSION}
  - # [WARNING!] This command wipes everything in your repo that is
  - # gitignored in preparation for the source release.
  - # You may want to check that there's nothing your care about here first.
  - # Alternatively you could clone the repo into another location as in
  - # git clone git@github.com:apache/incubator-superset.git superset-releases
  - git clean -fxd
  # Create the target folder
  mkdir -p ~/svn/superset_dev/${VERSION}/
  git archive \
  