Makefile: remove unused @@PERLLIBDIR@@ substitution variable

Makefile: remove unused @@PERLLIBDIR@@ substitution variable

Junio noticed that this variable is not quoted correctly when it is
passed to sed.  As a shell-quoted string, it should be inside
single-quotes like $(perllibdir_relative_SQ), not outside them like
$INSTLIBDIR.

In fact, this substitution variable is not used.  Simplify by removing
it.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>