Don't allow symbolic coefficients in in solvable graph

Author: YingboMa

src/structural_transformation/symbolics_tearing.jl

@@ -76,7 +76,7 @@ function eq_derivative!(ts::TearingState{ODESystem}, ieq::Int)
         add_edge!(s.graph, eq_diff, s.var_to_diff[var])
     end
     s.solvable_graph === nothing ||
-        find_eq_solvables!(ts, eq_diff; may_be_zero = true, allow_symbolic = true)
+        find_eq_solvables!(ts, eq_diff; may_be_zero = true, allow_symbolic = false)
 
     return eq_diff
 end