The blobStore.getBlobStoreManager().delete() API allows for deleting blob stores that are in use

Reproduce steps:

1. Fire up Nexus 3.13.0 with default configuration.
2. Go to system/tasks, and create a new execute script task.
3. For the script text use: blobStore.getBlobStoreManager().delete('default')
4. Run the script

Observe that the default blob store is deleted, even though it is in use by many repositories.

Expected:  The API should make the same validation checks the UI does, and prevent deleting blob stores that are in use by repositories.