[SL.Core] drop zero in favor of empty

doing this to be consistent with System.Drawing.Primitives

src/SixLabors.Core/Primitives/Point.cs

@@ -25,11 +25,6 @@ namespace SixLabors.Primitives
         /// </summary>
         public static readonly Point Empty = default(Point);
 
-        /// <summary>
-        /// Represents a <see cref="Point"/> that has X and Y values set to zero.
-        /// </summary>
-        public static readonly Point Zero = new Point(0, 0);
-
         /// <summary>
         /// Initializes a new instance of the <see cref="Point"/> struct.
         /// </summary>

src/SixLabors.Core/Primitives/PointF.cs

@@ -25,11 +25,6 @@ namespace SixLabors.Primitives
         /// </summary>
         public static readonly PointF Empty = default(PointF);
 
-        /// <summary>
-        /// Represents a <see cref="PointF"/> that has X and Y values set to zero.
-        /// </summary>
-        public static readonly PointF Zero = new PointF(0, 0);
-
         /// <summary>
         /// Initializes a new instance of the <see cref="PointF"/> struct.
         /// </summary>

src/SixLabors.Core/Primitives/Size.cs

@@ -23,11 +23,6 @@ namespace SixLabors.Primitives
         /// Represents a <see cref="Size"/> that has Width and Height values set to zero.
         /// </summary>
         public static readonly Size Empty = default(Size);
-        /// <summary>
-        /// Represents a <see cref="Size"/> that has Width and Height values set to zero.
-        /// </summary>
-        public static readonly Size Zero = new Size(0, 0);
-
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Size"/> struct.

src/SixLabors.Core/Primitives/SizeF.cs

@@ -24,11 +24,6 @@ namespace SixLabors.Primitives
         /// </summary>
         public static readonly SizeF Empty = default(SizeF);
 
-        /// <summary>
-        /// Represents a <see cref="SizeF"/> that has Width and Height values set to zero.
-        /// </summary>
-        public static readonly SizeF Zero = new SizeF(0, 0);
-
         /// <summary>
         /// Initializes a new instance of the <see cref="SizeF"/> struct.
         /// </summary>