Remove xlsfile references

[dave-mills]

It looks like the ODK Link Package still references xlsfile as a property of Xlsform and XlsformTemplate models, but we have moved to Spatie Media library collections to store the actual Excel files. This is causing at least one error (described below):

Context:

@dave-mills - I got issue when deploying the updated xlsform to ODK central...

Here are what I did:

1. get latest code from dev branch
2. run command "composer install", "npm i", "npm run build", "npm run dev"
3. run command "php artisan migrate:fresh --seed"
4. download the updated xlsform, add settings excel sheet to it
5. login as admin, create a new team, create xlsform template
6. admin panel > team resources > xlsforms relation manager, click "Add Xlsform to Team" button to add xlsform, error 500 occurred

I create another xlsform template with our previous xlsform, got same error.

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Screen shot:

Originally posted by @dan-tang-ssd in #37 (comment)