Vulkan: remove redundant function definitions in null_driver

When deprecating the old gen scripts, there's some duplicate entries get added,
which is fixed by this change.

This change also reorders the extension constants in the gen scripts so that
future modification is less likely to put duplicate entries.

Bug: 134185757
Test: ./scripts/code_generator.py && build
Change-Id: I6c8356b01e8a9c042293804eb628ed0a72da4a5c