Fix two issues with softmodem yaml files

Fix a typo that prevents casting max_chan to integer in nrue.uicc.yaml

Remove unused field ssb_PositionsInBurst_PR from gNB example config yaml file

Marking this as documentation as there is currently no testcase that uses rfsim modelling on the UE with the yaml file. EDIT: Keep as documentation even though this might actually fail tests because the test will run in the integration branch.