refactor: revert incorrect change to ui_client.c code

- There is no "resource leak".
- "return 0" is definitely not the correct behavior if we ever occur a
  platform where this would fail.
- There was no problem here to fix. so let's not "fix" it.
- once CI is upgraded to gcc 13, we'll figure out the correct way to make
  it shut the fuck up. warnings on non-ci platforms are not critical.

src/nvim/ui_client.c

@@ -65,11 +65,7 @@ uint64_t ui_client_start_server(int argc, char **argv)
 #ifdef MSWIN
     os_open_conin_fd();
 #else
-    int fd = dup(stderr_isatty ? STDERR_FILENO : STDOUT_FILENO);
+    dup(stderr_isatty ? STDERR_FILENO : STDOUT_FILENO);
-    if (fd < 0) {
-      return 0;
-    }
-    // FIXME: resource leak of fd
 #endif
   }