[Minuit2] Remove deleted header file reference

`math/minuit2/inc/Minuit2/FunctionMinimizer.h` was removed in 91bb4d7
but there is still a reference to `FunctionMinimizer.h` in
`math/minuit2/src/CMakeLists.txt`. This appears to be a simple
oversight, as similar references were removed in the aforementioned
commit. This is causing the build to fail, so this removes the
reference to fix the `Cannot find source file` error.

math/minuit2/src/CMakeLists.txt

@@ -33,7 +33,6 @@ set(MINUIT2_HEADERS
     FumiliStandardChi2FCN.h
     FumiliStandardMaximumLikelihoodFCN.h
     FunctionGradient.h
-    FunctionMinimizer.h
     FunctionMinimum.h
     GenericFunction.h
     GradientCalculator.h