Remove bits about kPoison in ObjPtr's documentation.

Follow-up CL to https://android-review.googlesource.com/342532.

Test: mmma art
Bug: 35644797
Change-Id: I86c2b74bd25e7d38a684bcb527300fb7e6e5ab29
diff --git a/runtime/obj_ptr.h b/runtime/obj_ptr.h
index 92cf4eb..4162873 100644
--- a/runtime/obj_ptr.h
+++ b/runtime/obj_ptr.h
@@ -29,7 +29,6 @@
 constexpr bool kObjPtrPoisoning = kIsDebugBuild;
 
 // Value type representing a pointer to a mirror::Object of type MirrorType
-// Pass kPoison as a template boolean for testing in non-debug builds.
 // Since the cookie is thread based, it is not safe to share an ObjPtr between threads.
 template<class MirrorType>
 class ObjPtr {