selftests: remove obsolete kconfig fragment for cpu-hotplug

Kconfig CONFIG_CPU_NOTIFIER_ERROR_INJECT has been removed since kernel 4.10
check commit:
    commit 530e9b76
    Author: Thomas Gleixner <tglx@linutronix.de>
    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 <Lei.Yang@windriver.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/cpu-hotplug/config

 CONFIG_NOTIFIER_ERROR_INJECTION=y
-CONFIG_CPU_NOTIFIER_ERROR_INJECT=m