Fix compile warning (#4186)

This was introduced after the last release, so no release notes needed.

com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs

                             tagsEqual = hitObject.CompareTag(tag);
                         }
                     }
-                    catch (UnityException e)
+                    catch (UnityException)
                     {
                         // If the tag is null, empty, or not a valid tag, just ignore it.
                     }