xtensa: remove KCORE_ELF again

The Kconfig symbol KCORE_ELF was removed in v2.6.0, but reappeared in two
architectures. It is useless. Remove it again.

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