Stop exporting objective and remove its doc

Author: YingboMa

docs/src/systems/OptimizationSystem.md

@@ -8,7 +8,7 @@ OptimizationSystem
 
 ## Composition and Accessor Functions
 
-- `get_op(sys)` or `objective(sys)`: The objective to be minimized.
+- `get_op(sys)`: The objective to be minimized.
 - `get_states(sys)` or `states(sys)`: The set of states for the optimization.
 - `get_ps(sys)` or `parameters(sys)`: The parameters for the optimization.
 - `get_constraints(sys)` or `constraints(sys)`: The constraints for the optimization.

src/ModelingToolkit.jl

@@ -202,7 +202,7 @@ export Equation, ConstrainedEquation
 export Term, Sym
 export SymScope, LocalScope, ParentScope, GlobalScope
 export independent_variables, independent_variable, states, parameters, equations, controls,
-       observed, structure, full_equations, objective
+       observed, structure, full_equations
 export structural_simplify, expand_connections, linearize, linearization_function
 export DiscreteSystem, DiscreteProblem