Makefile: remove redundant munging of @@INSTLIBDIR@@

Makefile: remove redundant munging of @@INSTLIBDIR@@

Junio originally added this in f6276fe159 for use in `unshift @INC,
'@@INSTLIBDIR@@'' in git-fmt-merge-msg.perl. That program was since
then rewritten in C in 00449f992b. And since 6fcca938b0 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: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>