Issue #693 - Remove `forecast_unit` argument from `get_duplicate_forecasts()`

[nikosbosse]

Description

This PR closes #693.

As described in #693. This PR removes the forecast_unit arg from

• get_duplicate_forecasts()
• check_duplicates() (which maybe should be check_duplicate_forecasts(), but then again it's internal anyway)

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [ ] I have added a news item linked to this PR.
• [ ] I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.31%. Comparing base (4463a1c) to head (3ead521).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #774 +/- ## ======================================= Coverage 95.31% 95.31% ======================================= Files 21 21 Lines 1602 1602 ======================================= Hits 1527 1527 Misses 75 75 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.