WebDriver: Remove active sessions from the close-the-session AO

These steps now have more than one caller; specifically, they may be
called from the Delete Session and Close Window endpoints. The session
was only removed from the active session map for the former endpoint.

Instead, let's more accurately handle removing the session where the
spec tells us to, so that all callers properly perform this step.

Userland/Services/WebDriver/Session.cpp

@@ -97,7 +97,7 @@ Web::WebDriver::Response Session::stop()
     web_content_connection().close_session();
 
     // 2. Remove the current session from active sessions.
-    // NOTE: Handled by WebDriver::Client.
+    m_client->close_session(session_id());
 
     // 3. Perform any implementation-specific cleanup steps.
     if (m_browser_pid.has_value()) {