[DOCS] Remove NON_SECURE_SVC_BOOT option from ARM Fast Model bootwrapper

The NON_SECURE_SVC_BOOT option was added in ARM Fast Model bootwrapper
to boot Xvisor ARM32 (without virtualization extensions) on VExpress-A15
emulated by ARM Fast Models.

Recent patch "[ARM] arm32: Ensure that we are in SVC mode before
turning-on MMU" makes Xvisor ARM32 to switch to SVC mode hence the
NON_SECURE_SVC_BOOT option in bootwrapper is now redundant.

This patch removes NON_SECURE_SVC_BOOT option from ARM Fast Model
bootwrapper.

Signed-off-by: Anup Patel <anup@brainfault.org>