For many git-based projects, submodules are useful in avoiding duplicate work and easing utility library updates.  There are times, however, when a submodule needs to be removed from a project.  Submodules aren't removed with git rm submoduledir, they must be removed in a more tedious, manual fashion.  There are many unclear explanations of how to remove a submodule but I found one on Stack Overflow that's concise, so I thought I'd share it.  The steps are as follows:

1. Delete the relevant section from the .gitmodules file.  The section would look similar to:

   [submodule "vendor"]
   
   	path = vendor
   
   	url = git://github.com/some-user/some-repo.git

2. Stage the .gitmodules changes via command line using:git add .gitmodules
3. Delete the relevant section from .git/config, which will look like:

   [submodule "vendor"]
   
   	url = git://github.com/some-user/some-repo.git

4. Run git rm --cached path/to/submodule .  Don't include a trailing slash -- that will lead to an error.
5. Run rm -rf .git/modules/submodule_name
6. Commit the change:
7. Delete the now untracked submodule files rm -rf path/to/submodule

Those steps will get you rid of that unwanted submodule.  A lot harder than adding one, eh?

from: http://davidwalsh.name/git-remove-submodule