Drop x11-themes/kde4-icons-oxygen in favor of x11-themes/kf5-oxygen-icons5.

Despite the name, kf5-oxygen-icons5 does not depend on Qt5 and just contains
the Oxygen icon set without any architecture-dependent files.

As such, we can make all ports depend on it and remove kde4-icons-oxygen
altogether and avoid having two conflicting ports for KDE4 and KF5.

x11-themes/kf5-oxygen-icons5 was adjusted to have its dependencies reduced even
further:

• The dependencies on qt5-core and qt5-testlib were removed, as they were only needed to build tests.
• We're also passing some CMake arguments to avoid looking for qt5-core altogether.

With the changes above, the only port kf5-oxygen-icons5 depends on at runtime
is devel/kf5-extra-cmake-modules, which itself does not depend on anything
else.

Reviewed by: tcberner
Differential Revision: https://reviews.freebsd.org/D10237