Remove multipacting attribute of RUN command

Follow up from https://gitlab.psi.ch/OPAL/Manual-2.1/issues/24#note_15340

The multipacting attribute of the RUN command is not documented in the manual (https://gitlab.psi.ch/OPAL/Manual-2.1/wikis/track), nor is there a regression test.

Is it still useful, or can it be deleted from the code completely?

Relevant lines in TrackRun.cpp:

    bool mpacflg = Attributes::getBool(itsAttr[MULTIPACTING]);
    if(!mpacflg) {
        if (Track::block->bunch->getIfBeamEmitting()) {
            Track::block->bunch->setChargeZeroPart(charge);
        } else {
            Track::block->bunch->setCharge(charge);
        }
        // set coupling constant
        double coefE = 1.0 / (4 * pi * epsilon_0);
        Track::block->bunch->setCouplingConstant(coefE);


        // statistical data are calculated (rms, eps etc.)
        Track::block->bunch->calcBeamParameters();
    } else {
        Track::block->bunch->setChargeZeroPart(charge);// just set bunch->qi_m=charge, don't set bunch->Q[] as we have not initialized any particle yet.
        Track::block->bunch->calcBeamParametersInitial();// we have not initialized any particle yet.
    }