1. LOGROTATEDIR is not used since commit f4e9a1df
("make: don't install service and logrotate configs").

2. SYSTEMDUNITDIR is not used since commit 10d5e9a4
("criu: scripts: remove criu service files").

3. install-tree target was *never* used, makes no sense
to keep it.

While at it, also
 - sort the variables in "export" statement to match
   the order of appearance in Makefile;

 - don't export DESTDIR (it is exported by default as
   it always comes from the make command line);

 - remove unused variable from INSTALL.md.

travis-ci: success for Makefile.install fixes
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>