test(common): remove PortReservation::release

With the archive tests on the shared fixture, no test releases a
reservation before a service binds its port. Remove the method so the
only way to start a service on a reserved address is to transfer the
bound listener, which keeps the address held for the child's lifetime.

Validation: `cargo check -p ngit-grasp --tests` finds no remaining
caller.

Assisted-by: Claude Fable 5.1
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/common/port.rs

@@ -16,8 +16,8 @@ use std::net::TcpListener;
 /// process can be handed the same number.
 ///
 /// Transfer it with [`PortReservation::into_std_listener`] to start a service
-/// without releasing its address. Dropping it or calling `release` relinquishes
+/// without releasing its address. Dropping it relinquishes the address, so a
-/// the address and must not be used before starting a replacement service.
+/// reservation must be transferred, not dropped, to start a service on it.
 #[derive(Debug)]
 pub struct PortReservation {
     port: u16,
@@ -59,14 +59,6 @@ impl PortReservation {
             _listener: listener,
         }
     }
-
-    /// Relinquish the address. A later bind to this port is inherently racy.
-    pub fn release(self) -> u16 {
-        let port = self.port;
-        // `self` is consumed; the listener inside is dropped here.
-        drop(self);
-        port
-    }
 }
 
 /// Bind `127.0.0.1:0`, capture the assigned port, and **keep the listener