kconfig: remove check_stdin()

Except silentoldconfig, valid_stdin is 1, so check_stdin() is no-op.

oldconfig and silentoldconfig work almost in the same way except that
the latter generates additional files under include/.  Both ask users
for input for new symbols.

I do not know why only silentoldconfig requires stdio be tty.

  $ rm -f .config; touch .config
  $ yes "" | make oldconfig > stdout
  $ rm -f .config; touch .config
  $ yes "" | make silentoldconfig > stdout
  make[1]: *** [silentoldconfig] Error 1
  make: *** [silentoldconfig] Error 2
  $ tail -n 4 stdout
  Console input/output is redirected. Run 'make oldconfig' to update configuration.

  scripts/kconfig/Makefile:40: recipe for target 'silentoldconfig' failed
  Makefile:507: recipe for target 'silentoldconfig' failed

Redirection is useful, for example, for testing where we want to give
particular key inputs from a test file, then check the result.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>