gdb/amdgpu: Silence wave termination messages

After commit 9d7d58e7262, 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 9d7d58e7262, 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>

gdb/amd-dbgapi-target.c

diff --git a/gdb/amd-dbgapi-target.c b/gdb/amd-dbgapi-target.c
index 22c269b..8eafcea 100644
--- a/gdb/amd-dbgapi-target.c
+++ b/gdb/amd-dbgapi-target.c
@@ -1634,7 +1634,7 @@ amd_dbgapi_target::update_thread_list ()
 	    auto it = threads.find (tp->ptid.tid ());
 
 	    if (it == threads.end ())
-	      delete_thread (tp);
+	      delete_thread_silent (tp);
 	    else
 	      threads.erase (it);
 	  }