x86/asm/32: Remove setup_once()

After the removal of the stack canary segment setup code, this function
does nothing.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20221115184328.70874-1-brgerst@gmail.com