Revert "Implement Caddy-Sponsors HTTP response header"

[jkrzefski]

This reverts commit 56453e9.

1. What does this change do, exactly?

It reverts the implementation of the Caddy-Sponsors HTTP response header.

2. Please link to the relevant issues.

56453e9

3. Which documentation changes (if any) need to be made because of this PR?

None.

4. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code
• I am willing to help maintain this change if there are issues with it later

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[francislavoie]

Duplicate of #1866, so I'm closing this one.