[ci] [R-package] speed up valgrind job

[jameslamb]

Proposes removing R-vignette-building from the valgrind CI job.

That job just installs the package and runs the unit tests:

LightGBM/.ci/test_r_package_valgrind.sh

Lines 14 to 19 in 2dfb9a4

	RDvalgrind \
	--no-readline \
	--vanilla \
	-d "valgrind --tool=memcheck --leak-check=full --track-origins=yes" \
	-f testthat.R \
	> ${ALL_LOGS_FILE} 2>&1 || exit -1

So the vignettes are totally unnecessary. Removing them removes the following sources of unnecessary work and therefore build time:

• the need to install {knitr} and its many recursive dependencies
• the extra compilation of lib_lightgbm done for vignette-building

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/7204236057

Status: failure ❌.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/7204258936

Status: failure ❌.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/7204287590

Status: failure ❌.

[jameslamb]

/gha run r-valgrind

Workflow R valgrind tests has been triggered! 🚀
https://github.com/microsoft/LightGBM/actions/runs/7296681094

Status: success ✔️.

[github-actions]

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