This was added when the index system was first built in order to
support testing out reindex without overwriting an existing index. I
don't think anybody is using it at present.
Since reindex is only ever run while the server is shut down, if an
admin really needs to point the reindexer at a different location,
they can edit the configured location in gerrit.config for index
implementations that are configured in this way. Alternatively, for
something like Lucene where the index just goes to a directory, they
can move the old directory out of the way to keep it as a backup.
We don't need yet another way of accomplishing this goal that also
clutters up the code.
Change-Id: Id5af3318d8a5e537be67e5953a6d43c180ab3095
5992220391