Revert "abook: fix compiling with gcc5"

This reverts commit 37918bdc7a09e34985c57a3fe64000edf92362b3.

has been fixed on master differently

pkgs/applications/misc/abook/default.nix

···
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ ncurses readline ];
 
-  # Changed inline semantics in GCC5, need to export symbols for inline funcs
-  postPatch = ''
-    substituteInPlace database.c --replace inline extern
-  '';
-
   meta = {
     homepage = "http://abook.sourceforge.net/";
     description = "Text-based addressbook program designed to use with mutt mail client";