remove masked_img return from custom_range threshold method

[joshqsumner]

Describe your changes
Removes the masked_img return from threshold.custom_range.

Type of update
This is code cleanup.

Associated issues
closes #1767

Additional context
Image can still be made with pcv.apply_mask if desired.

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

[deepsource-io]

DeepSource Code Review

We reviewed changes in 474b98a...73ebb00 on this pull request. Below is the summary for the review, and you can see the individual issues we found as inline review comments.

See full review on DeepSource ↗

PR Report Card

Overall Grade

Security   Reliability   Complexity   Hygiene   Coverage

Code Review Summary

Analyzer	Status	Updated (UTC)	Details
Python		Oct 9, 2026 1:54p.m.	Review ↗
Code coverage		Oct 9, 2026 1:54p.m.	Review ↗

Code Coverage Summary

Language	Line Coverage (New Code)	Line Coverage (Overall)
Aggregate	100% [✓ above threshold]	100%
Python	100% [✓ above threshold]	100%

➟ Additional coverage metrics may have been reported. See full coverage report ↗

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