fix(UserMountCache): Add back unique index for oc_mounts and use normal insert

[provokateurin]

Replaces https://github.com/nextcloud/server/pull/56929

With the unique index back there will be no more duplicate mounts, without modifying the transaction isolation level. Also to avoid any future issues with this method, I went ahead and removed all usage (it was deprecated in 15.0.0 already).

Oracle will likely fail on the truncate for the same still unknown reason as in https://github.com/nextcloud/server/pull/54014.