Remove erroneous ArgumentNullException mention from AsSpan (#7782)

Author: stephentoub

xml/System/MemoryExtensions.xml

@@ -651,8 +651,6 @@ Returns `default` when `text` is `null`.
         <summary>Creates a new read-only span over a portion of the target string from a specified position to the end of the string.</summary>
         <returns>The read-only span representation of the string.</returns>
         <remarks>To be added.</remarks>
-        <exception cref="T:System.ArgumentNullException">
-          <paramref name="text" /> is <see langword="null" />.</exception>
         <exception cref="T:System.ArgumentOutOfRangeException">
           <paramref name="start" /> is less than 0 or greater than <see langword="text.Length" />.</exception>
       </Docs>