scripts: Remove unused code from vk_helper.py
scripts: Remove unused code from vk_helper.py

Mostly the string_struct_helper related stuff, but also some
additional orphaned remnants.

Change-Id: Ibdd124d10351c694b02917c81e97c56dc9645e6b

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools