Revert "add to_vec for thunks"

This reverts commit 5d928d8.

Author: Miha Zgubic

src/to_vec.jl

@@ -193,9 +193,3 @@ function FiniteDifferences.to_vec(x::AbstractZero)
     end
     return Bool[], AbstractZero_from_vec
 end
-
-function FiniteDifferences.to_vec(t::Thunk)
-    v, back = to_vec(unthunk(t))
-    Thunk_from_vec = v -> @thunk(back(v))
-    return v, Thunk_from_vec
-end

test/to_vec.jl

@@ -176,10 +176,6 @@ end
             test_to_vec(ZeroTangent())
             test_to_vec(NoTangent())
         end
-
-        @testset "Thunks" begin
-            test_to_vec(@thunk(3.2+4.3))
-        end
     end
 
     @testset "FillVector" begin