[R-package] remove blank lines and not-really-comments from .Rbuildignore (#3318)

R-package/.Rbuildignore

+AUTOCONF_UBUNTU_VERSION
+^autom4te.cache/.*$
+^.*\.bin
 ^build_r.R$
-\.gitkeep$
-^docs/.*$
-^pkgdown$
 ^cran-comments\.md$
-
-# Objects created by compilation
-^.*\.o
-^.*\.so
+^docs/.*$
 ^.*\.dll
-^.*\.out
-^.*\.bin
-
-# Code copied in at build time
-^src/CMakeLists.txt$
+\.gitkeep$
 ^Makefile$
-^src/build/.*$
-^autom4te.cache/.*$
-
-# files only used during development
-AUTOCONF_UBUNTU_VERSION
+^.*\.o
+^.*\.out
+^pkgdown$
 ^recreate-configure\.sh$
-
-# unnecessary files from submodules
+^.*\.so
+^src/build/.*$
+^src/CMakeLists.txt$
 ^src/compute/.appveyor.yml$
 ^src/compute/.coveralls.yml$
 ^src/compute/.travis.yml$