Remove attribute @keepsOrientation in RTM4railML3.xsd

Links

• Forum discussion:
  • Christian Rahmig, 07.03.2022: https://www.railml.org/forum/index.php?t=msg&th=818&goto=2935&#msg_2935
• Trac tickets:
  • (RTM) railtopomodel#3 (closed)

Proposed solution in railML 3.2

The attribute @keepsOrientation shall be marked deprecated.