Removed obsoleted safe level checks on irb.

https://github.com/ruby/ruby/pull/1713

  Patch by @y-yagi [fix GH-1713]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60278 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/irb.rb

@@ -531,9 +531,6 @@ module IRB
             end
             print "Maybe IRB bug!\n" if irb_bug
           end
-          if $SAFE > 2
-            abort "Error: irb does not work for $SAFE level higher than 2"
-          end
         end
       end
     end