Remove internal uses of deprecated Orbit.from_body_ephem

Doing a quick search, the only place where we are using Orbit.from_body_ephem is this one:

https://github.com/poliastro/poliastro/blob/6ee8e9386a27d4d241cd0e9cb22f6fb6fbf2cf45/src/poliastro/plotting/_base.py#L372-L374

Instead, we should instead be doing this:

    ephem = Ephem.from_body(body, J2000_TDB)
    orbit = Orbit.from_ephem(Sun, ephem, J2000_TDB)

And, after that, tests covering Orbit.from_body_ephem can be removed.

[astrojuanlu]

Good catch @Yash-10 !

[astrojuanlu]

@ishanSrt The function has already been deprecated, there are no more changes needed to its function definition and docs.