Clean up CMake after BlueBrain/nmodl#1305

[matz-e]

With ode.py integrated into the C++ code, I don't think we need the PYTHONPATH handling any longer.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[azure-pipelines[bot]]

✔️ 7cdbf65bdad2fc1f7a7ac8e08ee17272ddb2ef33 -> Azure artifacts URL

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.25%. Comparing base (8ffa1fa) to head (7cdbf65).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2952 +/- ## ========================================== - Coverage 67.25% 67.25% -0.01% ========================================== Files 571 571 Lines 104902 104900 -2 ========================================== - Hits 70557 70551 -6 - Misses 34345 34349 +4 ```

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[bbpbuildbot]

Logfiles from GitLab pipeline #219650 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: mac_m1_cmake_build: [cmake, ON, OFF, OFF, address]
• :white_check_mark: spack_setup
• :white_check_mark: build:nmodl
• :white_check_mark: build:neuron:nmodl:intel:legacy
• :white_check_mark: build:neuron:nmodl:intel:shared
• :white_check_mark: build:neuron:nmodl:nvhpc:acc:legacy
• :white_check_mark: build:neuron:nmodl:nvhpc:acc:shared
• :white_check_mark: build:neuron:nmodl:nvhpc:omp:legacy
• :white_check_mark: build:neuron:nmodl:nvhpc:omp
• :white_check_mark: test:neuron:nmodl:intel:legacy
• :white_check_mark: test:neuron:nmodl:intel:shared
• :white_check_mark: test:neuron:nmodl:nvhpc:acc:legacy
• :white_check_mark: test:neuron:nmodl:nvhpc:acc:shared
• :white_check_mark: test:neuron:nmodl:nvhpc:omp:legacy
• :white_check_mark: test:neuron:nmodl:nvhpc:omp