Fix clean operation in fetch_sources.py
Fix clean operation in fetch_sources.py

Running fetch_sources.py with --clean parameter under Windows causes
script to fail, due to git creates files read only in following
folder: ".git\\objects\\pack\\*".

The fix now removes readonly files first, then the whole tree.

Components: Framework

VK-GL-CTS Issue: 661

Change-Id: I5f739405a090708d76642bdb4fa3623a25bd1dc7

KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS