remove old atan2 tests

[oxinabox]

I think when we switch atan2 tests to use frule_test and rrule_test we forgot to delete the old bad tests.

I think this is now tested well using

ChainRules.jl/test/rulesets/Base/base.jl

Lines 140 to 148 in 3e6eee3

	@testset "binary trig ($f)" for f in (hypot, atan)
	rng = MersenneTwister(123456)
	x, Δx, x̄ = 10randn(rng, 3)
	y, Δy, ȳ = randn(rng, 3)
	Δz = randn(rng)
	frule_test(f, (x, Δx), (y, Δy))
	rrule_test(f, Δz, (x, x̄), (y, ȳ))
	end