Remove backward compatible code from agent

We decided to disable the coroutine debugger for
versions of kotlinx-coroutines-core lower than 1.6
therefore the backward compatible path is not
necessary anymore.

Test: CoroutineDebuggerAgentTest
Bug: 182023904
Change-Id: I4f5702aaf15f7c75831fd7384532e2489bdec3bc
3 files changed