RFR(S): 8202070: Cleanup code after JDK-8200450, JDK-8200366

[Schmidt, Lutz]

Dear all, 

may I please request reviews for this cleanup change? 

Bug:    https://bugs.openjdk.java.net/browse/JDK-8202070 
Webrev: http://cr.openjdk.java.net/~lucy/webrevs/8202070.00/

This change removes debugging and tracing code unintentionally left in when JDK-8200450 was pushed. 

Besides the cleanup, very little additional checks were added to further harden the code. Most important are even more checks for pointer accessibility (is_readable_pointer()). I did only local (private) testing. Our nightbuild failed across all platforms for a reason not related to this change -> no test coverage tonight. Functional changes are minimal, so I take the risk of requesting your feedback now. 

Thank you!
Lutz