DfsGarbageCollector: avoid closing idx and bitmap streams twice

DfsGarbageCollector: avoid closing idx and bitmap streams twice

These try-with-resources blocks close the underlying output stream
twice: once when closing the CountingOutputStream wrapper, then again
when closing the DfsOutputStream out.

Simplify by only closing the CountingOutputStream.

In practice this shouldn't matter because the close() method of a
Closable is required to be idempotent, but avoiding the redundant
extra close makes the code simpler to read and understand.

Change-Id: I1778c4fc8ba075a2c6cd2129528bb272cb3a1af7