ANDROID: Drop explicit 'CONFIG_INIT_STACK_ALL_ZERO=y' from gki_defconfig

After a backport of commit dcb7c0b9 ("hardening: Clarify Kconfig
text for auto-var-init"), this becomes redundant, as
CONFIG_INIT_STACK_ALL_ZERO is enabled by default when the compiler
supports it, which results in savedefconfig dropping it.

Change-Id: Ia199fb999f98b936323cb60ecbbdc1ae365932c5
Signed-off-by: Nathan Chancellor <nathan@kernel.org>