Build definition is not removed in database even if you've already removed the build definition from the project/project group and all the build results that references it

Because of this, it is not possible to delete the schedule where this build definition is attached.

To reproduce this:
1. Create a new schedule named 'SCHEDULE_1'
2. Create a new build definition for your project group and attach this build definition to 'SCHEDULE_1'
3. Build the project group
4. Delete the build definition for the project group
5. Delete the build results that used this build definition
6. Delete 'SCHEDULE_1'

You will not be able to delete SCHEDULE_1 and get the following error:
'The schedule cant be removed, it is probably used by a build definition.'

If you look at the BUILDDEFINITION table, you will see that the build definition you created in step 2 is still there even though no project group/project/build result references it.

This only happens if you already used the build definition for building the project (e.g. a build result was created). Otherwise, the build definition is removed from the database.