ARC: Remove non existent refs to GENERIC_KERNEL_EXECVE & GENERIC_KERNEL_THREAD

This tracks mainline commit ae903caa "Bury the conditionals from
kernel_thread/kernel_execve series" which we missed out as ARC port was
not yet mainline.

[vgupta: commit log modified]

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>