diagnostic: remove MacGPG2 check

This hasn't been an issue for upstream stable releases for 3 years, since March
2013, and hasn't been an issue for pre-release versions for 4 years.

That release, and indeed the latest modern GPGTools releases, support 10.6 and
above so there's no reason to suspect people are actually encountering the old
versions in the wild with enough regularity to merit this being a permanent part
of our codebase any more.

In the last two years Homebrew has seen one Issue where MacGPG2 was the problem,
and that wasn't reproducible at the time (and still isn't), and the `doctor` check
likely wasn't even raised there. There has only been one Issue where the MacGPG2
`doctor` check was raised in that two year period.

I think it's fair to treat this as an user configuration outlier now rather than
an issue we need to be constantly on guard for.

Ref: Homebrew/legacy-homebrew@dfb171b
Ref: Homebrew/legacy-homebrew#12238
Ref: Homebrew/legacy-homebrew@046498b

Library/Homebrew/diagnostic.rb

@@ -114,25 +114,6 @@ module Homebrew
         EOS
       end
 
-      # Installing MacGPG2 interferes with Homebrew in a big way
-      # https://github.com/GPGTools/MacGPG2
-      def check_for_macgpg2
-        return if File.exist? "/usr/local/MacGPG2/share/gnupg/VERSION"
-
-        suspects = %w[
-          /Applications/start-gpg-agent.app
-          /Library/Receipts/libiconv1.pkg
-          /usr/local/MacGPG2
-        ]
-        return unless suspects.any? { |f| File.exist? f }
-
-        <<-EOS.undent
-          You may have installed MacGPG2 via the package installer.
-          Several other checks in this script will turn up problems, such as stray
-          dylibs in /usr/local and permissions issues with share and man in /usr/local/.
-        EOS
-      end
-
       # Anaconda installs multiple system & brew dupes, including OpenSSL, Python,
       # sqlite, libpng, Qt, etc. Regularly breaks compile on Vim, MacVim and others.
       # Is flagged as part of the *-config script checks below, but people seem