[Math][ROOT-10131] Fix gcc9 warnings (-Wdeprecated-copy) for Minuit2

- MnCross: Set default assignment operator
- MinosError: Set default assignment operator
- ABObj: Delete private assignment operator

math/minuit2/inc/Minuit2/ABObj.h

@@ -70,7 +70,7 @@ public:
 
 private:
 
-  ABObj& operator=(const ABObj&) {return *this;}
+  ABObj& operator=(const ABObj&) = delete;
 
 public:
 

math/minuit2/inc/Minuit2/MinosError.h

@@ -34,6 +34,8 @@ public:
 
    MinosError(const MinosError& err) : fParameter(err.fParameter), fMinValue(err.fMinValue), fUpper(err.fUpper),  fLower(err.fLower) {}
 
+   MinosError& operator=(const MinosError& ) = default;
+
    MinosError& operator()(const MinosError& err) {
       fParameter = err.fParameter;
       fMinValue = err.fMinValue;

math/minuit2/inc/Minuit2/MnCross.h

@@ -45,6 +45,8 @@ public:
 
   MnCross(const MnCross& cross) : fValue(cross.fValue), fState(cross.fState), fNFcn(cross.fNFcn), fValid(cross.fValid), fLimset(cross.fLimset), fMaxFcn(cross.fMaxFcn), fNewMin(cross.fNewMin) {}
 
+  MnCross& operator=(const MnCross&) = default;
+
   MnCross& operator()(const MnCross& cross) {
     fValue = cross.fValue;
     fState = cross.fState;