Revert "[Math] Reduce to the minimum the usage of R__LOAD_LIBRARY in tutorials"

This reverts commit 21d726ba.
This change is platform dependent! Everything works on ubuntu16, but fails
on other platform. The change is reverted for 6.13, waiting to understand more
about this difference.

tutorials/math/Bessel.C

@@ -33,6 +33,8 @@
 
 void Bessel()
 {
+   R__LOAD_LIBRARY(libMathMore);
+
    TCanvas *DistCanvas = new TCanvas("DistCanvas", "Bessel functions example", 10, 10, 800, 600);
    DistCanvas->SetFillColor(17);
    DistCanvas->Divide(2, 2);

tutorials/math/FeldmanCousins.C

@@ -14,6 +14,9 @@
 
 void FeldmanCousins()
 {
+   if (!gROOT->GetClass("TFeldmanCousins"))
+      R__LOAD_LIBRARY(libPhysics);
+
    TFeldmanCousins f;
 
    // calculate either the upper or lower limit for 10 observed

tutorials/math/Legendre.C

@@ -26,6 +26,8 @@
 
 void Legendre()
 {
+   R__LOAD_LIBRARY(libMathMore);
+
    TCanvas *Canvas = new TCanvas("DistCanvas", "Legendre polynomials example", 10, 10, 750, 600);
    Canvas->SetGrid();
    TLegend *leg = new TLegend(0.5, 0.7, 0.4, 0.89);

tutorials/math/Legendre.py

@@ -11,6 +11,7 @@
 
 import ROOT
 
+ROOT.gSystem.Load("libMathMore")
 Canvas = ROOT.TCanvas("DistCanvas", "Legendre polynomials example", 10, 10, 750, 600)
 Canvas.SetGrid()
 leg = ROOT.TLegend(0.5, 0.7, 0.4, 0.89)

tutorials/math/LegendreAssoc.C

@@ -34,6 +34,8 @@
 
 void LegendreAssoc()
 {
+   R__LOAD_LIBRARY(libMathMore);
+
    std::cout <<"Drawing associate Legendre Polynomials.." << std::endl;
    TCanvas *Canvas = new TCanvas("DistCanvas", "Associate Legendre polynomials", 10, 10, 800, 500);
    Canvas->Divide(2,1);