Revert "Merge pull request #1562 from woodruffw/check-admin-group"

This reverts commit 7f13b37b903564e2a64cea982d6b84d1aa6b800b, reversing
changes made to 62e0ecdaafbc6779205908c7ab191c884473afd6.

Library/Homebrew/diagnostic.rb

@@ -1069,19 +1069,6 @@ module Homebrew
         message
       end
 
-      def check_for_member_of_admin_group
-        groups = Utils.popen_read("groups").split
-        return unless groups.include?("admin")
-
-        <<-EOS.undent
-          You are not a member of the "admin" group, which will cause
-          commands like `brew linkapps` to fail.
-
-          To fix this, you can run:
-            `sudo dseditgroup -o edit -a $(whoami) -t user admin`
-        EOS
-      end
-
       def all
         methods.map(&:to_s).grep(/^check_/)
       end