Disable robots.txt generation.

[timcosgrove]

Description

Currently we output a simple default robots.txt file with the next-sitemap plugin. This PR disables that so that the existing robots.txt from Content Build continues to be used.

Ticket

#773 , indirectly. This PR does not close that ticket.

Developer Task

Tasks

Beta Give feedback

1. PR submitted against the main branch of next-build.
2. Link to the issue that this PR addresses (if applicable).
3. Define all changes in your PR and note any changes that could potentially be breaking changes.
4. PR includes steps to test your changes and links to these changes in the Tugboat preview (if applicable).
5. Alerted the #accelerated-publishing Slack channel to request a PR review.
6. You understand that once approved, you are responsible for merging your changes into main. (Note that changes to main will move automatically into production.)

Testing Steps

1. Go to the admin for the Tugboat instance associated with this PR: https://tugboat.vfs.va.gov/66fdd05e25a4ad7d7601e0bd
2. Enter 'Terminal' for the main web preview. This may take a second if the preview is currently suspended.
3. Run cd out && ls -al robots.txt

If the file is not found, the behavior is correct. We are removing the file with this PR.

If you see a file there and it has content like the following (less robots.txt), this PR is not passing:

# *
User-agent: *
Allow: /

# Host
Host: https://pr768-hrqyjsstuyk6vydpmua8bvdyp6fzfmci.tugboat.vfs.va.gov/

# Sitemaps
Sitemap: https://pr768-hrqyjsstuyk6vydpmua8bvdyp6fzfmci.tugboat.vfs.va.gov/sitemap.xml