fast-export: drop data parameter from anonymous generators

fast-export: drop data parameter from anonymous generators

The anonymization code has a specific generator callback for each type
of data (e.g., one for paths, one for oids, and so on). These all take a
"data" parameter, but none of them use it for anything. Which is not
surprising, as the point is to generate a new name independent of any
input, and each function keeps its own static counter.

We added the extra pointer in d5bf91fde44 (fast-export: add a "data"
callback parameter to anonymize_str(), 2020-06-23) to handle
--anonymize-map parsing, but that turned out to be awkward itself, and
was recently dropped.

So let's get rid of this "data" parameter that nobody is using, both
from the generators and from anonymize_str() which plumbed it through.
This simplifies the code, and makes -Wunused-parameter happier.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>