Remove OEM config for sim call manager [Telecom part]

This is the telecom part of deleting the OEM config.
See ag/710820 for the overlay part.

We can now set the default sim call manager using the following carrier config:
CarrierConfigManager.KEY_DEFAULT_SIM_CALL_MANAGER_STRING

This CL removes the old OEM config value.

BUG: 21499566
Change-Id: Ib287fa6d2c80b950fa1cb65e98c470fd7290c95c