Deleted variable mask=mask

[zeeuqsze]

Describe your changes
Removed variable that references itself

Type of update
Is this a:

• Update to documentation

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #1404 (82a2942) into main (36e1fe1) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1404   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          158       158           
  Lines         6939      6937    -2     
=========================================
- Hits          6939      6937    -2     

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
plantcv/plantcv/threshold/threshold_methods.py	100.00% <ø> (ø)