OvmfPkg/OvmfXen: Remove DEBUG_ON_SERIAL_PORT

Remove support for DEBUG_ON_SERIAL_PORT because OvmfXen can't be build
with it due to a circular dependency:
  DebugLib        : BaseDebugLibSerialPort ->
  SerialPortLib   : XenConsoleSerialPortLib ->
  XenHypercallLib : XenHypercallLib ->
  DebugLib

Also, if that dependency is fixed, I think it would be harder to find
which console the debug is sent to when running an HVM guest. The xen
console isn't the serial console used by default. Furthermore,
XenHypercallLib isn't initialised early enough, so we would loose
debug output from the SEC phase and early PEI phase.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200423095358.2518197-2-anthony.perard@citrix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>