ci: drop rdoc from the upstream jruby bundle #1083

Job	Run time
html5lib-tests	53s
jruby-head	45s
ruby-head (ubuntu, enable)	48s
ruby-head (ubuntu, disable)	45s
ruby-head-valgrind (disable)	5m 47s
ruby-head (windows, enable)	3m 40s
truffleruby-head (--disable-system-libraries --disable-static)	3m 12s
ruby-head (windows, disable)	3m 3s
truffleruby-head (--disable-system-libraries --enable-static)	2m 56s
ruby-head (macos, enable)	31s
xmlsoft-head (ubuntu)	2m 51s
truffleruby-head (--enable-system-libraries)	2m 23s
ruby-head (macos, disable)	36s
xmlsoft-head (windows)	9m 8s
xmlsoft-head (macos)	1m 38s
xmlsoft-head-valgrind	6m 40s
	45m 36s

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