src/meta-openembedded:libsdl2-mixer: drop deprecated modplug PACKAGECONFIG

libmodplug support has been removed in master branch
libsdl-org/SDL_mixer#673

Remove the option as we still provide libxmp to handle .mod

Signed-off-by: Markus Volk <f_l_kt-online.de>
Signed-off-by: Khem Raj <raj.khemgmail.com>

Author: MarkusVolk