treewide: remove unstable packages

There’s nothing wrong with unstable packages when they are maintained
and frequently updated. However, when they become out-of-date, as many
do, it is usually best to just get rid of them as the stable version
is become newer than the unstable version. This removes any packages
called "unstable" that have not been updated in over 1 year. Affected
packages include:

- isyncUnstable
- sxhkd-unstable
- dosbox-unstable

Revert "treewide: remove unstable packages"

This reverts commit df01b0b37a04624cfe6f8d2ccb42b99289841b68.

pkgs/tools/networking/isync/unstable.nix

@@ -1,31 +0,0 @@
-{
-  stdenv, fetchgit,
-  autoconf, automake, cyrus_sasl, db, openssl, perl, pkgconfig, zlib,
-}:
-
-stdenv.mkDerivation rec {
-  name = "isync-git-20170514";
-  rev = "4b3768806278a70db696ba52645dc1b6eb8de58a";
-
-  src = fetchgit {
-    url = "https://git.code.sf.net/p/isync/isync";
-    inherit rev;
-    sha256 = "1cv1isw01cfp7724z6f4pf6k4rx3k1lg0pc1xcq17zpikx9d10fb";
-  };
-
-  nativeBuildInputs = [ autoconf automake perl pkgconfig ];
-  buildInputs = [ cyrus_sasl db openssl zlib ];
-
-  preConfigure = ''
-    touch ChangeLog
-    ./autogen.sh
-  '';
-
-  meta = with stdenv.lib; {
-    homepage = http://isync.sourceforge.net/;
-    description = "Free IMAP and MailDir mailbox synchronizer";
-    license = licenses.gpl2Plus;
-    maintainers = with maintainers; [ the-kenny ttuegel ];
-    platforms = platforms.unix;
-  };
-}