RFR: 8197841: Remove unused function Universe::create_heap_ext

[Erik Helin]

Hi all,

this small patch removes an unused extension point, 
Universe::create_heap_ext. Since the definition of 
Universe::create_heap_ext is the only code in the file
src/hotspot/share/memory/universe_ext.cpp, I also removed that file.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8197841

Patch:
http://cr.openjdk.java.net/~ehelin/8197841/00/

Testing:
- `make run-test-tier1` on Linux x86-64

Thanks,
Erik