Fix valid storages removed when cleaning remote storages

The remote URL of a share is always stored in the database with a
trailing slash. However, when a cloud ID is generated trailing slashes
are removed.

The ID of a remote storage is generated from the cloud ID, but the
"cleanup-remote-storage" command directly used the remote URL stored in
the database. Due to this, even if the remote storage was valid, its ID
did not match the ID of the remote share generated by the command and
ended being removed.

Now the command generates the ID of remote shares using the cloud ID
instead, just like done by the remote storage, so there is no longer a
mismatch.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>