kconfig: qconf: drop Qt4 support

It is possible to keep this compatible with both Qt4 and Qt5, but it is
questionable if it is worth the efforts; it would require us to test
this on both of them, and prevent us from using new features in Qt5.

Qt5 was released in 2012, and now widely available.

Drop the Qt4 support.

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