[R-package] remove unused argument in early stopping callback

[jameslamb]

Noticed tonight that the function returned by cb.early.stop() has an argument finalize = FALSE.

That argument's value is never referenced, and no existing code expects to pass it. This PR proposes removing it.

The places where callbacks are executed only pass a single argument, env:

LightGBM/R-package/R/lgb.train.R

Lines 288 to 290 in 28c3c45

	for (f in cb$pre_iter) {
	f(env)
	}

LightGBM/R-package/R/lgb.train.R

Lines 329 to 331 in 28c3c45

	for (f in cb$post_iter) {
	f(env)
	}

LightGBM/R-package/R/lgb.cv.R

Lines 367 to 369 in 28c3c45

	for (f in cb$pre_iter) {
	f(env)
	}

LightGBM/R-package/R/lgb.cv.R

Lines 393 to 395 in 28c3c45

	for (f in cb$post_iter) {
	f(env)
	}

I'm also confident in this change because there are multiple unit tests checking that early stopping works correctly in the R package, for example:

LightGBM/R-package/tests/testthat/test_basic.R

Line 576 in 28c3c45

test_that("lgb.train() works with early stopping for classification", {

Notes for Reviewers

This is not a user-facing breaking change because all callbacks are internal to the package (since #2479 is still open).

[StrikerRUS]

LGTM!

[github-actions]

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