remove loadvoc CLI command and related tests

[osma]

This PR removes the annif loadvoc command which was deprecated in PR #614 but still available in the Annif 0.59 release but scheduled for removal in 0.59. The load-vocab command should be used instead.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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[codecov]

Codecov Report

Base: 99.61% // Head: 99.61% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (584b501) compared to base (ec10014).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
- Coverage   99.61%   99.61%   -0.01%     
==========================================
  Files          87       87              
  Lines        6038     5945      -93     
==========================================
- Hits         6015     5922      -93     
  Misses         23       23              

Impacted Files	Coverage Δ
annif/cli.py	99.67% <ø> (-0.02%)	⬇️
tests/test_cli.py	100.00% <ø> (ø)

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[osma]

All seems well in CI so I'll merge this.