Remove deprecated neural_net access from utils

[tvwenger]

What does this implement/fix? Explain your changes

This PR addresses the circular import issue described in #1301 by simply removing the deprecated code in sbi.utils.get_nn_models.py.

Does this close any currently open issues?

Fixes # #1301

Any relevant code examples, logs, error output, etc?

Now

from from sbi.neural_nets import posterior_nn

works as intended.

Any other comments?

Removing this deprecated code will break backwards-compatibility with users who still use the from sbi.utils import posterior_nn syntax.

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[janfb]

This makes sense, it's time to get rid of get_nn_models altogether. thanks for the fix!

Could you please also remove the test file at tests/neural_nets_factory.py testing the deprecation warning for get_nn_models?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.46%. Comparing base (18b1141) to head (d0a24e9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1302       +/-   ##
===========================================
- Coverage   89.30%   78.46%   -10.85%     
===========================================
  Files         119      118        -1     
  Lines        8725     8716        -9     
===========================================
- Hits         7792     6839      -953     
- Misses        933     1877      +944     

Flag	Coverage Δ
unittests	78.46% <ø> (-10.85%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sbi/utils/__init__.py	100.00% <ø> (ø)

... and 32 files with indirect coverage changes

[tvwenger]

Could you please also remove the test file at tests/neural_nets_factory.py testing the deprecation warning for get_nn_models?

Done @janfb!

[janfb]

Looks all good now. Many thanks for solving this nasty circular import problem 🙏