Remove Content-Security-Policy meta tag

[DL6ER]

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• What does this PR aim to accomplish?:

Remove Content-Security-Policy meta tag. We're setting this now as HTTP header which is the preferred way.

To be merged after pi-hole/pi-hole#4862 has been approved/merged.

• How does this PR accomplish the above?:

Remove the tag.

• What documentation changes (if any) are needed to support this PR?:

None

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[DL6ER]

Waiting on pi-hole/pi-hole#4862 to be merged, first.

[rdwebdesign]

@DL6ER

The header.php file was changed in devel branch, but the changes aren't showing in the diff.
I think this needs a local rebase.

[DL6ER]

This isn't important, the diff is only generated on opening or when a conflict arose. This PR can still be merged as is.