Deprecate `log` field in configuration files.

[desilinguist]

This PR closes #671.

• Remove temporary workaround to allow log instead of logs in the config files.
• Update relevant test to check for an error instead of a warning.
• Update all other tests that still relied on log field.

[codecov[bot]]

Codecov Report

Merging #705 (48c48c7) into main (f116e57) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #705      +/-   ##
==========================================
- Coverage   96.89%   96.89%   -0.01%     
==========================================
  Files          63       63              
  Lines        9213     9199      -14     
==========================================
- Hits         8927     8913      -14     
  Misses        286      286              

Impacted Files	Coverage Δ
skll/config/__init__.py	95.13% <ø> (-0.16%)	:arrow_down:
tests/test_classification.py	100.00% <ø> (ø)
tests/test_output.py	100.00% <ø> (ø)
tests/test_input.py	99.80% <100.00%> (-0.01%)	:arrow_down:
tests/test_regression.py	99.64% <100.00%> (ø)

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