kconfig: qconf: do not limit the pop-up menu to the first row

If you right-click the first row in the option tree, the pop-up menu
shows up, but if you right-click the second row or below, the event
is ignored due to the following check:

  if (e->y() <= header()->geometry().bottom()) {

Perhaps, the intention was to show the pop-menu only when the tree
header was right-clicked, but this handler is not called in that case.

Since the origin of e->y() starts from the bottom of the header,
this check is odd.

Going forward, you can right-click anywhere in the tree to get the
pop-up menu.

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