snow_depth seems not to be decreased when the snowpack melts, for snow packs without explicit layers

### Brief summary of bug

From tracing snow_depth through the code, I can't see anywhere where it is decreased when the snowpack melts. I expected to see such a decrease somewhere.

### General bug information

**CTSM version you are using:** ctsm1.0.dev074

**Does this bug cause significantly incorrect results in the model's science?** Yes (if it's actually a bug)

**Configurations affected:** All

### Details of bug

I'm not sure that this is actually a bug, but it seems suspicious to me and I'd like to get feedback from others. There are places where snow_depth is set to 0 (when the snowpack completely vanishes), and snow_depth is also recalculated in CombineSnowLayers, but other than that I can't find anywhere where snow_depth is decreased when the snowpack melts.

I expected to find code to decrease snow_depth in SnowCoverFractionSwensonLawrence2012Mod:UpdateSnowDepthAndFrac, but it looks like snow_depth can only *increase* there.