security: drop opener:allow-open-url now that nothing calls it

default.json carried this grant with an explicit accepted residual risk:
a compromised webview could make the OS open an attacker-chosen http(s)
URL. It was accepted because it could not be narrowed -- WebLinksAddon
opens links Claude printed inside the container, which are arbitrary by
construction, so a host allowlist would have deleted the feature.

Now that every host-browser open routes through `open_url_external`, the
webview has no reason to reach the plugin directly, and the risk closes
rather than stays recorded. The plugin remains a dependency: macOS and
Windows still use it, through `OpenerExt::open_url`, whose desktop
implementation calls `crate::open::open` directly and is not gated by
capabilities at all (tauri-plugin-opener-2.5.3/src/lib.rs:60) -- verified
rather than assumed, since the whole point is that the Rust path keeps
working. What is removed is the webview's ability to reach the opener
without passing the Rust-side validation.

The census note in default.json is rewritten to match, and lib.rs's
grant-list test is updated deliberately, as its own assertion message
demands.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src-tauri/src/lib.rs

@@ -938,7 +938,6 @@ mod tests {
             "core:webview:allow-internal-toggle-devtools",
             "dialog:allow-open",
             "dialog:allow-save",
-            "opener:allow-open-url",
         ];
         expected.sort();
         assert_eq!(