fs: remove uselib() system call

This system call has been deprecated for quite a while now.
Let's try and remove it from the kernel completely.

Link: https://lore.kernel.org/20250415-kanufahren-besten-02ac00e6becd@brauner


Acked-by: Kees Cook <kees@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>