removing from a collection mapped as a bag triggers a fetch of the whole collection

### Task List

- [x] Steps to reproduce provided
- [ ] Stacktrace (if present) provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce

1. run the Example Application
2. navigate to http://localhost:8080/test/index to trigger the issue for many-to-many
3. navigate to http://localhost:8080/test/index2 to trigger the issue for one-to-many

### Expected Behaviour

as per documentation, it is not expected that the whole collection is being fetched
see section 5.4.3 here: http://gorm.grails.org/6.1.x/hibernate/manual/#sets
"...adding to or removing from collections mapped as a Bag don’t trigger a load of all existing instances from the database..."

### Actual Behaviour

The whole locations collection is fetched when calling `post.removeFromLocations(location)`, which is very costly if there are thousand of items in the database.

### Environment Information

- **Operating System**: MacOS
- **GORM Version:** 6.1.11.RELEASE
- **Grails Version (if using Grails):** 3.3.9
- **JDK Version:** jdk1.8.0_121

### Example Application

- https://github.com/bp-FLN/gorm-removeFromBag-issue

Start by running the example application linked in the issue and reproduce the behavior at /test/index and /test/index2, focusing on post.removeFromLocations(location). Compare the observed collection fetch with the GORM 6.1 documentation section 5.4.3; done means removing an item from the bag does not fetch the entire collection.

Written by the indexing model from the issue text.