Fix the file extensions in .PNG files

Some .png files related to the Integrity documentation have a file
extension written with the capital letters. It does not work in Linux
environment.
Also, vas-galcore.PNG is not used at all in the Integrity documentation
so the file needs to be removed.

Change-Id: I56db691de19a36b6d2c7c826ab9419ef0e1390cb
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>