refactor: use new substitute_observed instead of tearing_substitute_expr

Author: AayushSabharwal

src/systems/systems.jl

@@ -147,7 +147,7 @@ function __mtkcompile(sys::AbstractSystem; simplify = false,
             is_scalar_noise = false
         end
 
-        noise_eqs = StructuralTransformations.tearing_substitute_expr(ode_sys, noise_eqs)
+        noise_eqs = substitute_observed(ode_sys, noise_eqs)
         ssys = System(Vector{Equation}(full_equations(ode_sys)),
             get_iv(ode_sys), unknowns(ode_sys), parameters(ode_sys); noise_eqs,
             name = nameof(ode_sys), observed = observed(ode_sys), defaults = defaults(sys),