CA-84305: Allow SR.destroy if an SR contains only unmanaged VDIs.

This was originally fixed as CA-79059, but the fix was lost when the
SR.allowed_operations code was moved to xapi_sr_operations.ml.

Signed-off-by: John Else <john.else@citrix.com>
Remove duplicated SR allowed_operations code.

Code has diverged slightly, but is still functionally identical.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1104 from johnelse/ca-83405-clearwater

CLEARWATER: CA-84305: Allow SR.destroy if an SR contains only unmanaged VDIs.