[R-package] remove blank lines and not-really-comments from .Rbuildignore

[jameslamb]

I saw this really interesting discussion on the r-pkg-devel mailing list: https://stat.ethz.ch/pipermail/r-package-devel/2020q3/005894.html

Basically, the .Rbuildignore file does not support comments. Lines that start with # will be evaluated as patterns that could match a file.

It also does not skip blank lines, so having blank lines just increases the time it takes to build a package.

This PR should slightly reduce the build time for the R package, and should avoid the unlikely but not impossible case where one of the "commented" lines matches and ignores a file that should not be ignored.

Since I'm touching this file, I also took the opportunity to rearrange it so that it is in alphabetical order (ignoring non-alphanumeric characters).

[StrikerRUS]

Maybe blank line after cran-comments.md in #3298 caused problems?

[jameslamb]

Maybe blank line after cran-comments.md in #3298 caused problems?

ooooo maybe! now that I know the exact R code that checks .Rbuildignore, I can probably come back in a follow-up PR and populate that file correctly. Might allow us to remove some rm statements from

LightGBM/build-cran-package.sh

Lines 33 to 35 in 1804fd1

	rm cran-comments.md
	rm AUTOCONF_UBUNTU_VERSION
	rm recreate-configure.sh

[github-actions]

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