Revert "libvirt: fix dlopen("libjansson.so.4")"

This reverts commit d46cfd2ff087816697f46d69f9f795ff878301c5, which
broke libvirt + some dependent packages (notably nixops) on Darwin

pkgs/development/libraries/libvirt/default.nix

···
     substituteInPlace src/lxc/lxc_conf.c \
       --replace 'lxc_path,' '"/run/libvirt/nix-emulators/libvirt_lxc",'
 
-    [ -f ${jansson}/lib/libjansson.so.4 ] || exit 1
-    substituteInPlace src/util/virjsoncompat.c \
-      --replace '"libjansson.so.4"' '"${jansson}/lib/libjansson.so.4"'
-
     patchShebangs . # fixes /usr/bin/python references
   '';