Makefile: use $(RMDIR) in magit-$(VERSION).tar{,.gz} targets

The "magit-$(VERSION).tar.gz" and "magit-$(VERSION).tar" targets should
remove the "magit-$(VERSION)" directory use $(RMDIR) instead of $(RM).

See commits d8c95a9d and 04e65579.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #833 from praet/makefile

Makefile: use $(RMDIR) in magit-$(VERSION).tar{,.gz} targets