Removed exceptions in SerialPort.IsOpen docs (dotnet#7441)

(Property System.IO.Ports.SerialPort.IsOpen is get-only, the documented exceptions make no sense)

Author: reijerh

xml/System.IO.Ports/SerialPort.xml

@@ -1556,8 +1556,6 @@
   
  ]]></format>
         </remarks>
-        <exception cref="T:System.ArgumentNullException">The <see cref="P:System.IO.Ports.SerialPort.IsOpen" /> value passed is <see langword="null" />.</exception>
-        <exception cref="T:System.ArgumentException">The <see cref="P:System.IO.Ports.SerialPort.IsOpen" /> value passed is an empty string ("").</exception>
       </Docs>
     </Member>
     <Member MemberName="NewLine">