This is a follow up to #126559. In that PR, strip data is never deleted even
when the strip that uses it is. Thus, the strip data array just keeps growing as
more strips are added, and never shrinks.
This PR implements that deletion. The approach is simple: remove the strip data
from the array using a swap-remove that swaps in the last item in the data
array, and then loop over the strips in the action to update any that were
referencing that swapped-in item. Additionally, before removal we check to make
sure the data item isn't still being used by any strips, and if it is then we
don't remove it.
Pull Request: https://projects.blender.org/blender/blender/pulls/127837
b200d1f975