Removed redundant flag from Image.Dispose(bool) call

As Image does not have unmanaged resources and does not implement finalizer method, there's no need for disposable pattern with a pair of Dispose() & Dispose(bool).

Due Dispose(bool) was changed to DisposeManaged().

src/ImageSharp/Image.cs

@@ -87,7 +87,7 @@ namespace SixLabors.ImageSharp
                 return;
             }
 
-            this.Dispose(true);
+            this.DisposeManaged();
 
             this.isDisposed = true;
         }
@@ -148,10 +148,9 @@ namespace SixLabors.ImageSharp
         protected void UpdateSize(Size size) => this.size = size;
 
         /// <summary>
-        /// Disposes the object and frees resources for the Garbage Collector.
+        /// Internal routine for freeing managed resources called from <see cref="Dispose"/>
         /// </summary>
-        /// <param name="disposing">Whether to dispose of managed and unmanaged objects.</param>
-        protected abstract void Dispose(bool disposing);
+        protected abstract void DisposeManaged();
 
         /// <summary>
         /// Throws <see cref="ObjectDisposedException"/> if the image is disposed.

src/ImageSharp/Image{TPixel}.cs

@@ -268,13 +268,7 @@ namespace SixLabors.ImageSharp
         }
 
         /// <inheritdoc/>
-        protected override void Dispose(bool disposing)
-        {
-            if (disposing)
-            {
-                this.frames.Dispose();
-            }
-        }
+        protected override void DisposeManaged() => this.frames.Dispose();
 
         /// <inheritdoc/>
         public override string ToString() => $"Image<{typeof(TPixel).Name}>: {this.Width}x{this.Height}";