bootstrap: don't do 'helm dependency update'

The intent of the final versions of !1999 (merged) was to avoid having to run helm dependency update during bootstrap kind cluster creation.

This was discussed in this thread specifically.

I just noticed today that this hadn't fully been done, because the step where we call helm dependency update in tools/kind/bootstrap-cluster.sh hadn't been removed.

This MR fixes the use case where bootstrap is done from an environment without Internet connectivity (so called "air-gapped").