[xbuild] Actually delete common files (CopyLocal) during Clean.

The common files that should be deleted during Clean (see 4c37deb)
weren't actually written to the CleanFile, as they were stored in
FileWrites after it was already used to create CombinedFileWrites.
Merge pull request #642 from Ventero/CleanCopyLocal

[xbuild] Actually delete common files (CopyLocal) during Clean.