test-bot: don't untap during cleanup_before

Library/Homebrew/cmd/test-bot.rb

@@ -662,7 +662,6 @@ module Homebrew
         safe_system "git", "checkout", "-f", "master"
         # This will uninstall all formulae, as long as
         # HOMEBREW_REPOSITORY == HOMEBREW_PREFIX, which is true on the test bots
-        Tap.names.each { |s| safe_system "brew", "untap", s if s != "homebrew/core" }
         safe_system "git", "clean", "-ffdx", "--exclude=/Library/Taps/" unless ENV["HOMEBREW_RUBY"] == "1.8.7"
       end
       pr_locks = "#{@repository}/.git/refs/remotes/*/pr/*/*.lock"