libdpkg: Remove unused pkglibdir variable from libdpkg.pc.in

The replaceable string @pkglibdir@ has possibly never been instantiated
from configure, but as the variable is unused, let's just remove it.

Warned-by: lintian

debian/changelog

@@ -71,6 +71,7 @@ dpkg (1.17.11) UNRELEASED; urgency=low
     cause problems when cross-building and using unmatched combinations of
     dpkg-dev and gcc. Closes: #751363
   * Update i386 architecture GNU cpu regex in cputable to match i786 too.
+  * Remove unused pkglibdir variable from libdpkg.pc.in.
 
   [ Updated programs translations ]
   * Danish (Joe Dalton). Closes: #754127

lib/dpkg/libdpkg.pc.in

@@ -2,7 +2,6 @@ prefix=@prefix@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
-pkglibdir=@pkglibdir@
 
 Name: libdpkg
 Description: Debian package management system library