Revert "Revert "Simplify full-core.gembox""

This reverts commit 37d795dec7.
This reapplies e20d652 (#4202), which is reverted, but its issue was solved
by 1d8456f.

mrbgems/full-core.gembox

@@ -1,9 +1,6 @@
 MRuby::GemBox.new do |conf|
-  conf.gem :core => "mruby-sprintf"
-  conf.gem :core => "mruby-print"
-
   Dir.glob("#{root}/mrbgems/mruby-*/mrbgem.rake") do |x|
     g = File.basename File.dirname x
-    conf.gem :core => g unless g =~ /^mruby-(print|sprintf|bin-debugger|test)$/
+    conf.gem :core => g unless g =~ /^mruby-(?:bin-debugger|test)$/
   end
 end