[9] RFR (XS) 8046607: Code cleanup: PerfMemory::backing_store_filename() should be removed

[Chris Plummer]

Hi,

Please review this trivial change to remove some unused code.

Summary: Removed PerfMemory::backing_store_filename() API since it is 
not used anywhere.

Webrev: http://cr.openjdk.java.net/~cjplummer/8046607/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8046607

Tested with:
     jprt "-testset hotspot"
     JTReg with hotspot tests and most JDK test
     JCK vm, lang, api
     vm.quick.testlist

thanks,

Chris