Remove test checking that TPOT-NN errors for multiclass input

[rachitk]

What does this PR do?

Removes or corrects tests erroring due to PR #1175 as they are defunct.

Where should the reviewer start?

Check tests/nn_tests.py

How should this PR be tested?

Check that all other tests pass besides the one removed.

Any background context you want to provide?

One test may still fail without further changes - one of the known_score tests (which may be due to scikit-learn updates). Left this one as a draft until that has been corrected, though this is an NN-specific change.

What are the relevant issues?

N/A

Screenshots (if appropriate)

N/A

Questions:

• Do the docs need to be updated? No.
• Does this PR add new (Python) dependencies? No.