[CRAN request]: Remove dependency on `leaflet.extras`

[ateucher]

We have asked for an update fixing the check problems shown at
https://cran.r-project.org/web/checks/check_results_leaflet.extras.html
with no successful update from the maintainer thus far.

Thus, package leaflet.extras is now scheduled for archival on
2026-02-19, and archiving this will necessitate also archiving its CRAN
strong reverse dependencies.

Please negotiate the necessary actions.

The CRAN Team

[ateucher]

This will be resolved by removing it from bcdata (bcgov/bcdata#363), it's not a direct dependency for bcmaps