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.
I think we cleared this up in IRC, but it's not new code. It was moved down from https://github.com/mongodb/mongo-php-driver/pull/794/files#diff-1545486dfe9dbfafb436652b73d79276L982, since I need to process `justOne` and set a limit before we start iterating on the cursor.