I assume that you are currently using Deduplication. So when Deduplication is enabled in CommVault it will write baseline data to the Mount Path (CHUNKs, Volumes and SFILES - Commvault way of writing data), which will be re-used if a similar reference is found.
The Mount Path then becomes associated to the Deduplication Policy.
If you are on version SP14 or later, Commvault has introduced in "Allocation Policy" -> Prevent data block references for new backups
"When selected, the deduplicated blocks in the mount path will not be referenced when there are multiple mount paths in the library. (Choose this option if you plan to retire the mount path.)"
Documentation - https://documentation.commvault.com/commvault/v11/article?p=9495.htm#o105501
The above purpose also works when you have losted the backend storage of the Mount Path.
By selecting the Option, you will inform Commvault to not reference Blocks on the Lost Mount Path, and Commvault will write new reference to other existing Mount Path.
Otherwise if you are prior to SP14, you might need to mark the current Mount Path "Disable for new writes" seal the associated DDB to the Mount Path. Then let CommVault age out the data, and once all referenced blocks are pruned from the Mount Path, this will also dis-associated the DDB. At this point you should be able to remove the Mount Path gracefully.