cd YourProject
git filter-branch --index-filter 'git rm --cached --ignore-unmatch -r YourProject.xcodeproj/project.xcworkspace' HEAD
# see what you want to do with your remote here...
# you can: git push origin master --force
# or you can delete it and push a fresh new one from your cleaned-up local...
rm -rf .git/refs/original
git gc --prune=now
git gc --aggressive --prune=now