revert logic to expose free_space, but keep availableStorage config

[phil-davis]

This is a copy of PR #40669 with just 1 commit and targeted to release-10.12.0

The original PR text was:

• fixes [QA] public links do not work with files_primary_s3 #40665
• related https://github.com/owncloud/enterprise/issues/5006 , allow specifying available space for objectstorages #40389
• needed for https://github.com/owncloud/metrics/pull/159

The issue here is that originally when doing #40389 I somehow mixed the storage_id for S3 bucket with the user urns storage_id's . So when matching with filecache I though it represents that one.

However I was wrong, and used the wrong storage .. also the correct filecache entry for the entire bucket is always showing 0 size.

In this PR, I just expose the config entry for availableStorage, so that apps could use it. Please note that this PR reverts logic to expose this on storage level, so all "quota" calculations wont work. If we want to make it work on storage level, something like background job that calculates this size would need to be done. However, metrics app does not require that complexity and it can simply just use that parameters as in https://github.com/owncloud/metrics/pull/159

[sonarcloud]

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[pako81]

nothing more to be done, right? merge this?