Remove load_trace and save_trace functions

[twiecki]

There are deprecation warnings about this for 2 years: #3902 We should remove them for 4.0.

[twiecki]

There are a few more here: #5109

[ariepratama]

hi @twiecki! I might be able to work on this issue 😄

do I need to add examples onto the documentation too? on how to save and load using arviz?