remove misleading comment in Rakefile

Comment introduced in bbb3e5a8 before rake `isolate:test` was even introduced bbb3e5a8

railties/Rakefile

@@ -8,9 +8,6 @@ require 'rbconfig'
 task :default => :test
 task :test => 'test:isolated'
 
-## This is required until the regular test task
-## below passes.  It's not ideal, but at least
-## we can see the failures
 namespace :test do
   task :isolated do
     dir = ENV["TEST_DIR"] || "**"