rbd: don't specify allow-shrink flag

rbd: don't specify allow-shrink flag

It was introduced by commit 4b7dd9d ("allow --allow-shrink on RBD
resize"), but doesn't give a rationale. A mail gives more[0],
indicating that the user also uses the function to shrink images.
However, the volume_resize function is only reachable via the resize
API endpoints for VMs and containers, which have an explicit check to
disallow shrinkage. If somebody really wants to shrink the image, just
let them use the storage's tools directly. Calling into Proxmox VE's
perl functions directly is not supported.

[0]: https://lists.proxmox.com/pipermail/pve-devel/2016-November/024077.html

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>