Remove ppl

Move ppl to homebrew-versions as it depends on gmp4.

Closes Homebrew/homebrew#17917.

Formula/ppl.rb

@@ -1,16 +0,0 @@
-require 'formula'
-
-class Ppl < Formula
-  homepage 'http://bugseng.com/products/ppl/'
-  url 'http://bugseng.com/products/ppl/download/ftp/releases/1.0/ppl-1.0.tar.gz'
-  sha1 '5f543206cc9de17d48ff797e977547b61b40ab2c'
-
-  depends_on 'gmp'
-
-  def install
-    system "./configure", "--disable-debug", "--disable-dependency-tracking",
-                          "--prefix=#{prefix}",
-                          "--enable-optimization=sspeed"
-    system "make install"
-  end
-end