kconfig: remove stale lxdialog/.gitignore

When this .gitignore was added, lxdialog was an independent hostprogs-y.

Now that all objects in lxdialog/ are directly linked to mconf, the
lxdialog is no longer generated.

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