fix: make NN ensemble suggest operations silent

[osma]

This one-line PR makes NN ensemble suggest operations silent. They used to be quiet at least in Annif 0.57, but apparently something changed in TensorFlow and now there are messages like this:

1/1 [==============================] - 0s 15ms/step 
1/1 [==============================] - 0s 15ms/step 
1/1 [==============================] - 0s 15ms/step

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #620 (e0ca692) into master (48b23f7) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #620   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          87       87           
  Lines        6034     6038    +4     
=======================================
+ Hits         6011     6015    +4     
  Misses         23       23           

Impacted Files	Coverage Δ
annif/backend/nn_ensemble.py	99.29% <ø> (ø)
tests/test_vocab.py	100.00% <0.00%> (ø)

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