Revert "vmm: Deprecate MemoryZoneConfig::file"

This reverts commit 9fb0274479.

A user was identified of this functionality.

See: #4837
Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/config.rs

@@ -2009,10 +2009,6 @@ impl VmConfig {
 
         if let Some(zones) = &self.memory.zones {
             for zone in zones.iter() {
-                if zone.file.is_some() {
-                    warn!("MemoryZoneConfig::file is deprecated and will be removed in future release (#4837)");
-                }
-
                 let id = zone.id.clone();
                 Self::validate_identifier(&mut id_list, &Some(id))?;
             }