Compatibility ggplot2 3.6.0

[teunbrand]

Hi there,

Apologies for not posting an issue first.
The ggplot2 package is planning an update for around May 2025 and a reverse dependency test identified a problem with the agricolaeplotr package.
The details are explained in tidyverse/ggplot2#6290, but essentially ggplot2 doesn't populate the plot$labels field before plot building anymore, which violates some test assumptions in this package.

The tests that were checking for the length of the labels field were fragile (easy to break by even mild upstream changes), so this PR removes them.
You can test the changes yourself with the development version of ggplot2 (pak::pak("tidyverse/ggplot2"))

Best,
Teun

[piusdahinden]

Dear teunbrand,
Thank you for your contribution. I've checked the changes. It seems to work fine.
Kind regards,
Pius