Remove unused code btif::core::btif_dut_mode_cback am: 4e2a7ca2 am: 47ef58a0

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Bluetooth/+/2944687



Change-Id: I34f3111a457049a0b1012a62b52c6e8a5f73dd52
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>