refactor: remove duplicate/redundant condition

[apermo]

Context

• Removed second condition with single_month_title( ' ', false ) in WPSEO_Replace_Vars::retrieve_date() as this did not provide any difference.
• This solves Function single_month_title() called twice in the same line in WPSEO_Replace_Vars::retrieve_date #21975

Summary

This PR can be summarized in the following changelog entry:

• Simplifies calculating the date replacement variable before displaying it in the frontend. Props to apermo.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

Check the impact on the date replacement variable

• Go to Yoast SEO->Settings->Date archives and confirm that the SEO title has the date replacement variable
• Visit the example.com/2024 and example.com/2024/12 date archives (make sure you have at least one post published on December 2024)
• Compare the SEO title of both between this RC and the production version and confirm it's the same

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #21975

[leonidasmi]

CR + Acceptance test is ✅