RFR(S): 8157428: [JVMCI] remove MemoryAccessProvider.readUnsafeConstant from API

[Roland Schatz]

Hi,

Please review this small cleanup change:
http://cr.openjdk.java.net/~rschatz/JDK-8157428/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8157428

This just changes a public method to package-private (and moves the 
JavaDoc over, no reason to delete it just because it's no longer public 
API).
This should really have been in the recent cleanup (JDK-8156835), but 
somehow it slipped through.

Thanks,
Roland