If you do not have any changes to your workbook (you can hide them first), there may be an easier way:
find -type d -name .svn -delete git add -u git commit -m 'remove svn folders -- no idea which maniac would stage them'
If you want to only disable them, but not physically delete them, go with anton answer :
find -type d -name .svn -print0 | xargs -0 git rm -r --cached
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