x11-toolkits/mygui: Remove obsolete functions from build (+)

Library builded with obsolete API support leads to build error for some consumers, e.g. for OpenMW:

ld: error: undefined symbol: non-virtual thunk to MyGUI::Widget::getLayerItemByPoint(int, int) const

referenced by widgets.cpp

widgets.cpp.o:(vtable for MWGui::Widgets::MWSkill) in archive apps/openmw/libopenmw-lib.a

referenced by widgets.cpp

widgets.cpp.o:(vtable for MWGui::Widgets::MWAttribute) in archive apps/openmw/libopenmw-lib.a

referenced by widgets.cpp

widgets.cpp.o:(vtable for MWGui::Widgets::MWSpell) in archive apps/openmw/libopenmw-lib.a

referenced 44 more times

While here, adopt the port

Approved by: amdmi3 (former maintainer, via PM)