kconfig: remove unused code for S_DEF_AUTO in conf_read_simple()

[ Upstream commit 92d4fe0a ]

The 'else' arm here is unreachable in practical use cases.

include/config/auto.conf does not include "# CONFIG_... is not set"
line unless it is manually hacked.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Stable-dep-of: a409fc14 ("kconfig: fix memory leak in sym_warn_unmet_dep()")
Signed-off-by: Sasha Levin <sashal@kernel.org>