Make IsFallbackCulture private

I can't see how this is used externally?

Author: angularsen

UnitsNet/CustomCode/UnitSystem.cs

@@ -157,9 +157,8 @@ public UnitSystem() : this(DefaultCulture)
 
         /// <summary>
         /// Whether this instance is for the <see cref="FallbackCulture"/>.
-        /// TODO Make this private.
         /// </summary>
-        public bool IsFallbackCulture => Culture.Equals(FallbackCulture);
+        private bool IsFallbackCulture => Culture.Equals(FallbackCulture);
 
         /// <summary>
         /// Clear the cached singleton instances.