Remove discontinued `ftpprd.ncep.noaa.gov` download paths

[blaylockbk]

Discussed in https://github.com/blaylockbk/Herbie/discussions/440

^{Originally posted by mtjonesy May 26, 2025} National Centers for Environmental Prediction (NCEP) Central Operations (NCO) will discontinue https access to the following websites: https://ftpprd.ncep.noaa.gov https://ftp.ncep.noaa.gov Identical data is available via the HTTPS protocol at https://nomads.ncep.noaa.gov/pub Full memo here: https://www.weather.gov/media/notification/pdf_2025/scn25-35_discontinue_https_access_for_FTPPRD.pdf

[blaylockbk]

A quick search shows that only the GFS model template has ftpprod urls.

https://github.com/blaylockbk/Herbie/blob/69be3cd13840b00ccf97bef4fc66d74f25bb925e/src/herbie/models/gfs.py

These should be removed.

[blaylockbk]

Duplicate of #427