.drop to .drop_vars

[ctuguinay]

Fixes part of #1309

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.52%. Comparing base (9f56124) to head (1f02cfe).
Report is 131 commits behind head on main.

Files	Patch %	Lines
echopype/calibrate/range.py	33.33%	2 Missing ⚠️
echopype/calibrate/calibrate_ek.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1364      +/-   ##
==========================================
+ Coverage   83.52%   89.52%   +6.00%     
==========================================
  Files          64       11      -53     
  Lines        5686     1002    -4684     
==========================================
- Hits         4749      897    -3852     
+ Misses        937      105     -832     

Flag	Coverage Δ
unittests	89.52% <50.00%> (+6.00%)	⬆️

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[ctuguinay]

@leewujung This is ready to review. It fixes a part of #1309.

[leewujung]

Thanks @ctuguinay , LGTM!