Summary:
Now that files scheduled for deletion are kept in the same directory, we don't need to constrain deletion scheduler to `db_paths[0]`. Previously this was done because there was a separate trash directory, and this constraint prevented files from being accidentally copied to another filesystem when they're scheduled for deletion.
Closes https://github.com/facebook/rocksdb/pull/3544

Differential Revision: D7093786

Pulled By: ajkr

fbshipit-source-id: 202f5c92d925eafebec1281fb95bb5828d33414f