This commit adds a new `WP_REST_Server#remove_header` method and uses it to clear the `Last-Modified` header when the "no caching" headers are sent (by default for all authenticated REST API requests). This matches the behavior of the `nocache_headers` function used in other parts of WordPress.
Previously, the REST API would send an empty `Last-Modified` header in this situation. Under some server and browser configurations, this causes browsers to cache authenticated REST API requests, which is undesirable.
Props iv3rson76, zinigor, rmccue, jnylen0.
Fixes#40444.
git-svn-id: https://develop.svn.wordpress.org/trunk@40805 602fd350-edb4-49c9-b593-d223f7449a82