powerpc/nvram_64: Remove unused code

nvram_find_partition() has no user. The call site was removed in the
arch/powerpc move, but the function stayed. Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>