Assuming you have a submodule named foo in the root of your repo:
git submodule deinit -f -- foo
rm -rf .git/modules/foo
git rm -f submodule
If that was the last or only modules you can also remove .gitmodules:
rm .gitmodules
Then just commit and push as usual.
If for some reason you have your submodules in subdirectories then add
the relative path to the subdirectory in front of the submodule name
(path/to/foo).
#git #tasks