remove snapshot test for default error

[simonpcouch]

I'm not sure this snap tests anything that we're interested in:

parsnip/tests/testthat/_snaps/boost_tree.md

Lines 43 to 47 in 4562458

	Code
	translate(boost_tree(formula = y ~ x))
	Condition
	Error in `boost_tree()`:
	! unused argument (formula = y ~ x)

Noticed this in the transition to snapshot testing in #1190.

[simonpcouch]

Turns out there are quite a few of these. Let's address #1197, #1198, and the following in one swoop:

parsnip/tests/testthat/_snaps/nullmodel.md

Lines 20 to 24 in 4562458

	Code
	translate(null_model(formula = y ~ x))
	Condition
	Error in `null_model()`:
	! unused argument (formula = y ~ x)

[simonpcouch]

parsnip/tests/testthat/_snaps/rand_forest.md

Lines 45 to 49 in 4562458

	Code
	translate(rand_forest(mode = "classification", ytest = 2))
	Condition
	Error in `rand_forest()`:
	! unused argument (ytest = 2)

[github-actions]

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