contrib/subtree/Makefile: scrap unused $(gitdir)

In 7ff8463dba, the references to gitdir
were removed but the assignment itself wasn't. Hence, drop the gitdir
assignment.

Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: James Denholm <nod.helm@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -4,7 +4,6 @@
 prefix ?= /usr/local
 mandir ?= $(prefix)/share/man
 libexecdir ?= $(prefix)/libexec/git-core
-gitdir ?= $(shell git --exec-path)
 man1dir ?= $(mandir)/man1
 
 gitver ?= $(word 3,$(shell git --version))