debuggerd: don't send SIGSTOP to crashing processes.

This was actually nonfunctional until f5e8f0b9, because it was using kill
after privileges were dropped. This doesn't seem necessary after the
changes to the sibling thread ptrace logic, though.

Bug: http://b/27427439
Change-Id: I6bffbc14e0cf5e377bbfa39c945518e0d436c223
(cherry picked from commit b17f228f)