I needed to do this recently, and I just ran the following command from my root directory of my repo:
find . -name '.svn' | xargs git rm -rf --ignore-unmatch
This search is performed recursively for all occurrences of the .svn folder and recursively deletes it and its contents. the --ignore-unmatch prevents git from being held if it does not find the specified file in the repository.
The next thing to do, of course, is to add .svn to your .gitnore so that you do not mistakenly start tracking these files again.
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