Clean up the following duplicated headers set by nginx

[hnarayanan]

It is likely that our nginx configuration duplicates what is already set in some baseline configuration. Not only are they now repeated, some of them set conflicting values.

Strict-Transport-Security: max-age=15768000
Strict-Transport-Security: max-age=31536000
Strict-Transport-Security: max-age=60; includeSubDomains; preload

Vary: Accept-Encoding
Vary: Accept-Language

X-Content-Type-Options: nosniff
X-Content-Type-Options: nosniff

X-Frame-Options: DENY
X-Frame-Options: SAMEORIGIN

X-XSS-Protection: 1; mode=block
X-XSS-Protection: 1; mode=block

This list can be re-generated and any fixes can be verified with the following command:

curl --head https://scancer.org | sort