ns: drop custom reference count initialization for initial namespaces

ns: drop custom reference count initialization for initial namespaces

Initial namespaces don't modify their reference count anymore.
They remain fixed at one so drop the custom refcount initializations.

Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-16-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>