remove save_trace and load_trace function

[ariepratama]

related to #5108

pymc is going to completely remove save_trace and load_trace functions. These functionalities will be replaced by arviz library.

For example save_trace should be replaced by arviz.to_netcdf (https://arviz-devs.github.io/arviz/api/generated/arviz.to_netcdf.html)

[codecov]

Codecov Report

Merging #5123 (7c7a1a1) into main (ace2589) will decrease coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5123      +/-   ##
==========================================
- Coverage   77.83%   77.61%   -0.22%     
==========================================
  Files          87       87              
  Lines       14008    13981      -27     
==========================================
- Hits        10903    10852      -51     
- Misses       3105     3129      +24     

Impacted Files	Coverage Δ
pymc/backends/ndarray.py	70.30% <ø> (-16.68%)	⬇️
pymc/__init__.py	96.07% <100.00%> (ø)
pymc/backends/__init__.py	100.00% <100.00%> (ø)

[twiecki]

Thanks @ariepratama!