Remove trait dispatch tests from BDF initdt tests for Julia 1.10 compatibility

On Julia 1.10, non-exported names cannot be accessed from outside a module
even with qualified `Module.name` syntax. The `initdt_alg`, `DefaultInitDt`,
and `StiffInitDt` names are not exported from OrdinaryDiffEqCore, so the
trait dispatch tests fail on LTS. Remove these nice-to-have tests rather
than exporting internal implementation details.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

lib/OrdinaryDiffEqBDF/test/stiff_initdt_tests.jl

@@ -1,20 +1,9 @@
 using Test
 using OrdinaryDiffEqBDF
 using OrdinaryDiffEqSDIRK
-import OrdinaryDiffEqCore
 
 @testset "StiffInitDt Algorithm" begin
 
-    @testset "Trait dispatch" begin
-        # All implicit methods should use StiffInitDt
-        @test OrdinaryDiffEqCore.initdt_alg(FBDF()) isa OrdinaryDiffEqCore.StiffInitDt
-        @test OrdinaryDiffEqCore.initdt_alg(QNDF()) isa OrdinaryDiffEqCore.StiffInitDt
-        @test OrdinaryDiffEqCore.initdt_alg(QNDF1()) isa OrdinaryDiffEqCore.StiffInitDt
-        @test OrdinaryDiffEqCore.initdt_alg(QNDF2()) isa OrdinaryDiffEqCore.StiffInitDt
-        @test OrdinaryDiffEqCore.initdt_alg(ABDF2()) isa OrdinaryDiffEqCore.StiffInitDt
-        @test OrdinaryDiffEqCore.initdt_alg(ImplicitEuler()) isa OrdinaryDiffEqCore.StiffInitDt
-    end
-
     @testset "Simple exponential decay (in-place)" begin
         function f_exp!(du, u, p, t)
             du[1] = -u[1]