MuonCalibMath - Use std::legendre function instead of custom made one.

Remove custom made Legendre polynomials, given that there's a std::legendre equivalent. Plus the singelton pattern does not make sense. I've checked that the coefficients of the current implementation are matching what's in Wikipedia and also what's reported in the cpp standard.

c++

 switch(k) {
        case 0:
            return 1;
            break;
        case 1:
            return x;
            break;
        case 2:
            return 0.5*(3.0*x*x-1.0);
            break;
        case 3:
            return 0.5*(5.0*x*x*x-3.0*x);
            break;
        case 4:
            return (35.0*std::pow(x, 4)-30.0*x*x+3.0)/8.0;
            break;