ART: Remove suspension from AttachAgent

It conflicts with calls later. Every plugin needs to do local
protections for itself.

Follow-up to commit ad2dc875.

Bug: 31682382
Bug: 34275017
Test: m test-art-host-run-test-909-attach-agent
Change-Id: Ib698d880feb774a00c819ef14c27a48501ca1f13