qemu: domain: Drop declaration of qemuDomainDefValidateDisk

There is no such function in our code. Commit abca72faa4 added it
spuriously.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>

src/qemu/qemu_domain.h

@@ -888,9 +888,6 @@ bool qemuDomainVcpuHotplugIsInOrder(virDomainDefPtr def)
 void qemuDomainVcpuPersistOrder(virDomainDefPtr def)
     ATTRIBUTE_NONNULL(1);
 
-int qemuDomainDefValidateDisk(const virDomainDiskDef *disk,
-                              virQEMUCapsPtr qemuCaps);
-
 int qemuDomainCheckMonitor(virQEMUDriverPtr driver,
                            virDomainObjPtr vm,
                            qemuDomainAsyncJob asyncJob);