Revert "Add check for HEAD ref in diagnostics"

Library/Homebrew/diagnostic.rb

@@ -756,7 +756,7 @@ module Homebrew
         end
       end
 
-      def check_coretap_git_config
+      def check_coretap_git_origin
         coretap_path = CoreTap.instance.path
         return if !Utils.git_available? || !(coretap_path/".git").exist?
 
@@ -785,16 +785,6 @@ module Homebrew
               git -C "#{coretap_path}" remote set-url origin #{Formatter.url("https://github.com/Homebrew/homebrew-core.git")}
           EOS
         end
-
-        head = coretap_path.git_head
-        return if head.nil? || head =~ %r{refs/heads/master}
-
-        <<-EOS.undent
-          Homebrew/homebrew-core is not on the master branch
-
-          Check out the master branch by running:
-            git -C "$(brew --repo homebrew/core)" checkout master
-        EOS
       end
 
       def __check_linked_brew(f)