Don't prepopulate legend key sizes

[teunbrand]

This PR aims to fix #6339.

Briefly, it no longer prepopulates legend.key.width and legend.key.height from the global theme and instead uses the default path of computing these, which includes the local theme.

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
big_keys <- theme(legend.key.size = unit(3, "cm"))

ggplot(mpg, aes(displ, hwy, colour = drv)) +
  geom_point() +
  guides(colour = guide_legend(theme = big_keys))

^{Created on 2025-02-19 with reprex v2.1.1}

[teunbrand]

Since we haven't prepopulated these, we now just use a <rel> object to inherit a taller bar from legend.key.size.

[thomasp85]

LGTM