Don't generate a nonce

[etiennebarrie]

The need for a nonce was removed in 3ba21de.

Rails needs to generate the nonce to put in the headers, even if nothing uses it later.

Before:

$ curl localhost:3000/maintenance_tasks -sD - | grep Content-Security-Policy                                                                                                                                                           
Content-Security-Policy: base-uri 'none'; default-src 'self'; object-src 'none'; script-src 'sha256-NiHKryHWudRC2IteTqmY9v1VkaDUA/5jhgXkMTkgo2w='; frame-ancestors 'self'; block-all-mixed-content; style-src https://cdn.jsdelivr.net 'sha256-y9V0na/WU44EUNI/HDP7kZ7mfEci4PAOIjYOOan6JMA=' 'nonce-J0XZEshlYJMsNi23H63z0w=='

After:

$ curl localhost:3000/maintenance_tasks -sD - | grep Content-Security-Policy
Content-Security-Policy: base-uri 'none'; default-src 'self'; object-src 'none'; script-src 'sha256-NiHKryHWudRC2IteTqmY9v1VkaDUA/5jhgXkMTkgo2w='; frame-ancestors 'self'; block-all-mixed-content; style-src https://cdn.jsdelivr.net 'sha256-y9V0na/WU44EUNI/HDP7kZ7mfEci4PAOIjYOOan6JMA='