simplify format_col.default, allowing e.g. vctrs_list_of columns to print well

[MichaelChirico]

Closes #5948

Turns out, this code in support of #2273 (#5224) is not needed -- I don't notice any difference before/after this change and we pass the related test:

data.table/inst/tests/other.Rraw

Lines 220 to 226 in a36caac

	if (loaded[["sf"]]) { #2273
	DT = as.data.table(st_read(system.file("shape/nc.shp", package = "sf"), quiet=TRUE))
	test(15, DT[1:3, .(NAME, FIPS, geometry)], output="Ashe.*-81.4.*Surry.*-80.4")
	dsf = sf::st_as_sf(data.table(x=1:10, y=1:10, s=sample(1:2, 10, TRUE)), coords=1:2)
	test(16, split(dsf, dsf$s), list(`1` = dsf[dsf$s == 1, ], `2` = dsf[dsf$s == 2, ]))
	}

It's possible our testing is just not extensive enough.

[github-actions]

Generated via commit 867773f

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 41 seconds
Installing different package versions	7 minutes and 34 seconds
Running and plotting the test cases	2 minutes and 12 seconds

[MichaelChirico]

This test fails:

registerS3method("format", "foo2130", function(x, ...) rep("All hail foo",length(x)))
test(2130.15, print(DT), output="All hail foo")  # e.g. sf:::format.sfc rather than sf:::format.sfg on each item

My sense is that if we only fail a toy example, we should just break it. In general I am thinking the better solution here is to add format_col and/or format_list_item methods as needed.

I think given the potential for breaking change, it's best to save this PR for 1.18.0.