Fix (again) high memory usage in optimize command

[osma]

Fixes #236

[codecov]

Codecov Report

Merging #243 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
+ Coverage   99.21%   99.21%   +<.01%     
==========================================
  Files          48       48              
  Lines        2177     2179       +2     
==========================================
+ Hits         2160     2162       +2     
  Misses         17       17

Impacted Files	Coverage Δ
annif/cli.py	99.4% <100%> (ø)	⬆️

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