Assume_excess_ice_streams off for clm6_0 cases when user gives an finidat file (and on for clm4_5 or clm5_0)

Now that we have a long history of doing clm6_0 simulations with excess ice on, we can turn off the stream files for the any cases that have finidat files to startup from.

@mvdebolskiy this is from my understanding of how excess ice works. You should ONLY use the streams file when you don't have excess ice on the IC file. In cases where you do have excess ice -- you should always use it from there. And this is also true for spinup cases like 1850 compsets. The reason is that there could be a few gridcells where excess ice melts from the previous simulation, and if that's the case you want that excess ice to remain melted.

I'm sure about this for 2000, 2010 and HIST compsets, I'm just less sure about 1850, since those cases run for hundreds to thousands of years. So in those spinup cases it's more likely that some random anomalous warming could happen and melt excess ice that maybe should have?