kconfig: remove k_invalid from expr_parse_string() return type

The only possibility of k_invalid being returned was when
expr_parse_sting() parsed S_OTHER type symbol. This actually never
happened, and this is even clearer since S_OTHER has gone.

Clean up unreachable code.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>