Revert "Fix double addition to sorted list (#1916)"

This reverts commit 964ab96.

Author: sebastienlagarde

com.unity.render-pipelines.core/CHANGELOG.md

@@ -57,7 +57,6 @@ The version number for this package has increased due to a version update of a r
 - Fix hierarchicalbox gizmo outside facing check in symetry or homothety mode no longer move the center
 - Fix artifacts on Adreno 630 GPUs when using ACES Tonemapping
 - Fixed a null ref in the volume component list when there is no volume components in the project.
-- Fixed issue with volume manager trying to access a null volume.
 
 ### Changed
 - Restored usage of ENABLE_VR to fix compilation errors on some platforms.

com.unity.render-pipelines.core/Runtime/Volume/VolumeManager.cs

@@ -124,8 +124,7 @@ public void Register(Volume volume, int layer)
             // Look for existing cached layer masks and add it there if needed
             foreach (var kvp in m_SortedVolumes)
             {
-                // We add the volume to sorted lists only if the layer match and if it doesn't contain the volume already.
-                if ((kvp.Key & (1 << layer)) != 0 && !kvp.Value.Contains(volume))
+                if ((kvp.Key & (1 << layer)) != 0)
                     kvp.Value.Add(volume);
             }