rusty-gd: Remove CancelableTaskTracker

Pairing, connection and A2DP streaming works with rusty-gd.

Also, tested with the default flags to make sure I am not breaking
anything.

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost DirectHciTest

Change-Id: I325e6aba146d9351aa7a06ab3c847cd410a502f3