Deprecate downloads from spedas.org in mms_load_*_segments load routine

[jameswilburlewis]

The default in these routines is to pull the segment time CSV files directly from the MMS SDC, but Eric also used to maintain IDL .sav files covering the whole mission on spedas.org. These files need to be updated periodically, but haven't been touched since Jan 2022. We can probably just eliminate the spedas.org option, rather than worry about an every-six-month task slipping through the cracks...

[jameswilburlewis]

The burst segment loader is fixed, but the others will need additional updates to be able to pull directly from the MMS SDC.