chore: delete old assets when uploading

[whd]

rsync with -d looks something like this (also added -n for this output):

Building synchronization state...
Starting synchronization...
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==> NOTE: You are performing a sequence of gsutil operations that may
run significantly faster if you instead use gsutil -m rsync ... Please
see the -m section under "gsutil help options" for further information
about when gsutil -m can be advantageous.

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