gdb/amdgpu: Silence wave termination messages

After commit 9d7d58e7, the amdgpu target started printing
"thread exited" messages when pruning waves that had terminated.

  ...
  [AMDGPU Wave ?:?:?:2045 (?,?,?)/? exited]
  [AMDGPU Wave ?:?:?:2046 (?,?,?)/? exited]
  [AMDGPU Wave ?:?:?:2047 (?,?,?)/? exited]
  [AMDGPU Wave ?:?:?:2048 (?,?,?)/? exited]
  ...

The issue was that before commit 9d7d58e7, delete_thread was silent
by default due to a bug that the commit fixed.

Replaced the amdgpu target call to delete_thread with a call to
delete_thread_silent.

Change-Id: Ie5d5a4c5be851f092d2315b2afa6a36a30a05245
Approved-By: Simon Marchi <simon.marchi@efficios.com>