svnmap.txt was added in a single revision and was not changed anymore.

commit 533c79dc67824d95b0aff5a252f9415d3bc80716
Author: Antoine Pitrou <solipsis@pitrou.net>
Date:   Sat Mar 5 17:46:03 2011 +0100

    Add the map of SVN revision numbers to changeset ids.
    This is mostly to help history-digging, especially when
    focussing on feature branches.
    Note that not all changesets are in the main repository,
    but they are all in the full history repository.

It is needed for history. Old commit messages and discussion on mailing lists refer to Subversion revisions. Without svnmap.txt you will lost these references unless you have a clone of Subversion repository. I hope you are not going to get rid of Misc/HISTORY?

It would be better to create a mapping from Subversion revisions to Git hashes.