Remove auspice-client/customisations/splash.js

[victorlin]

nextstrain.org now uses its own dataset listing UI, so this file can probably be removed.

Tasks:

• /fetch URLs can still end up at the auspice splash page
• URLs containing "narrative" can still end up at the auspice splash page
• failure to GET /charon/getDataset can end up at the Auspice splash page
• remove the file once it can no longer be accessed

(some tasks from #310 (comment))

[tsibley]

I added another way to get to the Auspice splash page to the task list above, e.g.