L2CAP ClassicLink: Don't delete channel before scheduler

The order of data members should be consistent with their dependency.
The Scheduler depends on ClassicFixedChannelAllocator because the
pointers inside the Scheduler points to channels in ChannelAllocator.

On dtor, the members are destructed in reverse order. We must delete the
Scheduler first.

Test: bluetooth_test_gd and cert/run_cert.sh
Change-Id: I2ec8c8de26dbb8741e15f57744f352ecb58aca8a