Revert "test-bot: clean up on Jenkins before building PR"

This reverts commit 51e4e6490bda0d981afd1787a6392ab588b0601c. This did
not work as intended, as the `git` command is executed in the context of
the tap the current job operates on, never for Homebrew/homebrew itself.

Library/Homebrew/cmd/test-bot.rb

@@ -285,7 +285,6 @@ module Homebrew
         @url = ENV["ghprbPullLink"]
         @hash = nil
         test "git", "checkout", "origin/master"
-        test "git", "clean", "-qxdff", "--exclude=/Library/Taps/"
       # Use Travis CI pull-request variables for pull request jobs.
       elsif travis_pr
         @url = "https://github.com/#{ENV["TRAVIS_REPO_SLUG"]}/pull/#{ENV["TRAVIS_PULL_REQUEST"]}"