Strip srcref from the $constructor expression

[yjunechoe]

Fixes #6467 - layer expression shows up correctly on print

p_expr <- parse(text = "
  p <- ggplot() +
    geom_point(size = 5)
")
eval(p_expr)

p$layers[[1]]$constructor
#> geom_point(size = 5)

I don't think I see existing tests for $constructor and this is only a minor display change, but if we were to add a test for the PR it'd be something like:

expect_identical(
  capture.output(p$layers[[1]]$constructor),
  "geom_point(size = 5)"
)

[teunbrand]

This looks good to me, thanks for the contribution!