arm64: Kconfig.debug: Remove unused CONFIG_DEBUG_ERRORS

The Kconfig entry for DEBUG_ERRORS is a verbatim copy of the former arm
entry for that symbol. It got removed in v2.6.39 because it wasn't
actually used anywhere. There are still no users of DEBUG_ERRORS so
remove this entry too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
[catalin.marinas@arm.com: removed option from defconfig]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>