kconfig: do not clear SYMBOL_VALID when reading include/config/auto.conf

When conf_read_simple() is called with S_DEF_AUTO, it is meant to read
previous symbol values from include/config/auto.conf to determine which
include/config/* files should be touched.

This process should not modify the current symbol status in any way.
However, conf_touch_deps() currently invalidates all symbol values and
recalculates them, which is totally unneeded.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>