`geom_polygon()` has useless `lineend` parameter

[teunbrand]

In addition to geom_polygon(), also other closed shape layers like geom_rect() have this parameter.
As these shapes are closed, there is no lineend to be drawn anywhere.

library(ggplot2)

ggplot(data.frame(x = c(1, 2, 2, NA), y = c(0, 0, 1, 1))) + 
  geom_polygon(
    aes(x, y),
    colour = "black", fill = 'grey50', linewidth = 5,
    lineend = "square"
  )

^{Created on 2024-10-16 with reprex v2.1.1}

I think we should just throw the ignoring unknown parameter warning for these.

[teunbrand]

After some fooling around, I no longer think this is worth fixing:

• It currently isn't doing any harm, it just isn't observed by closed shapes.
• Changing the Geom$draw_*() signature to remove lineend can hurt extensions that directly pass arguments to such methods.
• It'd invoke a lot of 'invisible' visual test updates, as the svg file will no longer contain the stroke-linecap: butt attribute in rectangles and polygons.