Remove deprecated argument to build_explicit_observed_function

Author: BenChung

src/systems/diffeqs/odesystem.jl

@@ -445,8 +445,7 @@ function build_explicit_observed_function(sys, ts;
         eval_module = @__MODULE__,
         output_type = Array,
         checkbounds = true,
-        drop_expr = drop_expr,
-        ps = parameters(sys),
+                ps = parameters(sys),
         return_inplace = false,
         param_only = false,
         op = Operator,