RFR(s): 8214181: safepoint header cleanup

[Kim Barrett]

> On Nov 21, 2018, at 9:17 AM, Robbin Ehn <robbin.ehn at oracle.com> wrote:
> 
> Hi all, please review.
> 
> A cleanup of unused headers in safepoint.hpp, which have some minor collateral effects.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8214181
> 
> Webrev:
> http://cr.openjdk.java.net/~rehn/8214181/webrev/index.html
> 
> Passes t1, locally built without precompiled headers.
> 
> Thanks, Robbin

Looks good.