qemu: fix memory leak in blockinfo
    
    Coverity flagged commit 0282ca45 as introducing a memory leak;
    in all my refactoring to make capacity probing conditional on
    whether the image is non-raw, I missed deleting the unconditional
    probe.
    
    * src/qemu/qemu_driver.c (qemuStorageLimitsRefresh): Drop
    redundant assignment.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>