Update code and written guidance in "Reading ICESat-2 Data in for Analysis" tutorial

[asteiker]

https://icepyx.readthedocs.io/en/latest/example_notebooks/IS2_data_read-in.html

Remove references to variable discovery.

[JessicaS11]

Remove references to variable discovery.

Only necessary for any cases where that discovery would use CMR. Currently, the Read module is primarily a tool for reading in local data (too slow in the cloud), so all the variable actions that happen are based on local files. I've been working really hard over the last year or so to help folks understand that this functionality is available (and am currently working on solutions to keep it available), so I want to be really careful about blanket removing all variables content.