x86: Remove CONFIG_4KSTACKS

These days 4 kilobytes of stack just aren't enough for reliably operation,
and people using lots of threads have long switched to 64-bit kernels, so
remove the CONFIG_4KSTACKS option.

Signed-off-by: Christoph Hellwig <hch@lst.de>
LKML-Reference: <20100628121614.GB6605@lst.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>