CopyChecker.checkCopy uses check_permission badly

CopyChecker.checkCopy recently gained a check to assist in the perpetuation of the ubuntu-security permission special case. It is bad for three reasons: 1) check_permission isn't to be used in model code, nor code that takes a user explicitly for permission checking -- and checkCopy falls in both categories. 2) It always checks for launchpad.Append on the primary archive, rather than the archive being copied to. 3) The hack is only needed in Archive.syncSource(s), so it should not plague the rest of the codebase.