Makefile: remove redundant munging of @@INSTLIBDIR@@

Junio originally added this in f6276fe1 for use in `unshift @inc,
'@@INSTLIBDIR@@'' in git-fmt-merge-msg.perl. That program was since
then rewritten in C in 00449f99. And since 6fcca938 all Perl
programs use `use lib' to set their @inc path.

There's been no @@INSTLIBDIR@@ in any Perl script to replace since
then. So there's no reason to keep it around.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1545,11 +1545,10 @@ $(patsubst %.perl,%,$(SCRIPT_PERL)): % : %.perl
 	sed -e '1{' \
 	    -e '	s|#!.*perl|#!$(PERL_PATH_SQ)|' \
 	    -e '	h' \
-	    -e '	s=.*=use lib (split(/$(pathsep)/, $$ENV{GITPERLLIB} || "@@INSTLIBDIR@@"));=' \
+	    -e '	s=.*=use lib (split(/$(pathsep)/, $$ENV{GITPERLLIB} || "'"$$INSTLIBDIR"'"));=' \
 	    -e '	H' \
 	    -e '	x' \
 	    -e '}' \
-	    -e 's|@@INSTLIBDIR@@|'"$$INSTLIBDIR"'|g' \
 	    -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' \
 	    $@.perl >$@+ && \
 	chmod +x $@+ && \