Back out dirty reference tracking, as the design was flawed.

Back out dirty reference tracking, as the design was flawed.

Objects can be written to the journal but not to the cloud--for example, if
a data block is written to the journal but overwritten before the file is
flushed to the cloud.  This write-combining is good, but the old code for
tracking when a journal segment could be reclaimed couldn't handle this.

So, back out that dirty reference tracking code, in preparation for
replacing it with another approach.