Finally starting to prune away some of the old fields that have
been made redundant by the new fields, on my way towards directly
reusing virStorageSource.

* src/util/virstoragefile.h (_virStorageFileMetadata): Drop
field.
* src/util/virstoragefile.c (virStorageFileGetMetadataInternal)
(virStorageFileChainLookup): Adjust callers.
* tests/virstoragetest.c (_testFileData, testStorageChain)
(mymain): Simplify test.
Signed-off-by: NEric Blake <eblake@redhat.com>