Remove obsolete allow

The lowest version of libc that devicemapper can use is 0.2.155. The
makedev function is no longer unsafe in any version of libc that
devicemapper can depend on transitively.

Signed-off-by: mulhern <[email protected]>

src/core/device.rs

@@ -85,10 +85,7 @@ impl From<Device> for dev_t {
             )
         }
         #[cfg(not(target_os = "android"))]
-        #[allow(unused_unsafe)] // No longer unsafe in libc 0.2.133.
-        unsafe {
-            makedev(dev.major, dev.minor)
-        }
+        makedev(dev.major, dev.minor)
     }
 }