Removing submodule manually involves number of steps and this worked for me.
Assuming you are in the project root directory and sample git module name is "c3-pro-ios-framework"
Remove the files associated to the submodule
rm -rf .git/modules/c3-pro-ios-framework/
Remove any references to submodule in config
vim .git/config
Remove .gitmodules
rm -rf .gitmodules
Remove it from the cache without the "git"
git rm --cached c3-pro-ios-framework
Add submodule
git submodule add https://github.com/chb/c3-pro-ios-framework.git