The argument to remove() is a filter document, so passing in an empty document means ‘remove all’:
db.user.remove({})
However, if you definitely want to remove everything you might be better off dropping the collection. Though that probably depends on whether you have user defined indexes on the collection i.e. whether the cost of preparing the collection after dropping it outweighs the longer duration of the remove() call vs the drop() call.
More details in the docs.