PHP-1229: Fix file/chunk deletion for GridFS::remove() and justOne

https://jira.mongodb.org/browse/PHP-1229

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Previously, deleting a single file (with justOne) that was selected by some non-unique criteria could result in chunks from multiple files being deleted (because the initial files query was not limited).

Additionally, the final file deletion (after chunk removal) would re-use the original criteria. There is no guarantee that the same file would be matched in the second query, which could result in GridFS::remove() deleting file A's chunks but file B's file document.
Why do you use > 0 instead of != 0 ? or rather just "if (justOne)" ?