selftests: remove obsolete kconfig fragment for cpu-hotplug

Kconfig CONFIG_CPU_NOTIFIER_ERROR_INJECT has been removed since kernel 4.10
check commit:
commit 530e9b76ae8f863dfdef4a6ad0b38613d32e8c3f
Author: Thomas Gleixner <[e-mail]>
Date: Wed Dec 21 20:19:53 2016 +0100

cpu/hotplug: Remove obsolete cpu hotplug register/unregister
functions

hotcpu_notifier(), cpu_notifier(), __hotcpu_notifier(),
__cpu_notifier(),
register_hotcpu_notifier(), register_cpu_notifier(),
__register_hotcpu_notifier(), __register_cpu_notifier(),
unregister_hotcpu_notifier(), unregister_cpu_notifier(),
__unregister_hotcpu_notifier(), __unregister_cpu_notifier()
<snip>

Signed-off-by: Lei Yang <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/cpu-hotplug/config

···
 CONFIG_NOTIFIER_ERROR_INJECTION=y
-CONFIG_CPU_NOTIFIER_ERROR_INJECT=m