kconfig: nconf: get rid of (void) casts from wattrset() calls

This reverts commit 10175ba6 ("nconfig: Silence unused return values
from wattrset").

With this patch applied, recent GCC versions can cleanly build nconf
without "value computed is not used" warnings.

The wattrset() used to be implemented as a macro, like this:

  #define wattrset(win,at) \
          (NCURSES_OK_ADDR(win) \
            ? ((win)->_attrs = NCURSES_CAST(attr_t, at), \
               OK) \
            : ERR)

The GCC bugzilla [1] reported a false-positive -Wunused-value warning
in a similar test case. It was fixed by GCC 4.4.1.

Let's revert that commit, and see if somebody will claim the issue.

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39889



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