Remove temporary workaround in ArviZ backend

[michaelosthege]

I noticed this line https://github.com/pymc-devs/pymc/blob/main/pymc/backends/arviz.py#L116-L118
stating that there's a temporary workaround that can be removed after the ArviZ 0.11.2 release which is available now.

Versions and main components

• PyMC3 Version: main

[OriolAbril]

also update requirement file to ask for arviz>=0.11.3