Remove incorrect exception comment from AsSpan (#66051)

This method never throws an `ArgumentNullException` for a null `text` input, and is even annotated as accepting `string?`.

src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs

@@ -101,7 +101,6 @@ public static ReadOnlySpan<char> AsSpan(this string? text)
         /// </summary>
         /// <param name="text">The target string.</param>
         /// <param name="start">The index at which to begin this slice.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is null.</exception>
         /// <exception cref="System.ArgumentOutOfRangeException">
         /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
         /// </exception>