RFR: 8197842: Remove unused macros VM_STRUCTS_EXT and VM_TYPES_EXT

[Erik Helin]

Hi all,

this patch removes the unused extension marcos VM_STRUCTS_EXT and 
VM_TYPES_EXT. Since these macros are the only content of 
vmStructs_ext.hpp, this patch also removes the file vmStructs_ext.hpp.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8197842

Webrev:
http://cr.openjdk.java.net/~ehelin/8197842/00/

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

Thanks,
Erik