xtensa: no need to select USE_GENERIC_SMP_HELPERS

Commit f615136c

 ("xtensa: add SMP support") added "select
USE_GENERIC_SMP_HELPERS". But the Kconfig symbol USE_GENERIC_SMP_HELPERS
was already removed in v3.13, so that select is a nop. Drop it.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>