perl DB_file: fix build by not changing $out/lib/perl5

The package already installs into $oul/lib/perl5/site_perl by itself.
Presumably it broke after d0bad145f5 due to removing perllocal.pod
(because this compiles on master and not here).

pkgs/development/perl-modules/DB_File/default.nix

@@ -16,12 +16,4 @@ buildPerlPackage rec {
     INCLUDE = ${db}/include
     EOF
   '';
-
-  # I don't know about perl paths, but PERL5LIB env var is managed through
-  # lib/perl5/site_perl, and the *.pm should be inside lib/perl5/site_perl/...
-  # for other packages to get that in the PERL5LIB env var.
-  postInstall = ''
-    mkdir -p $out/lib/perl5/site_perl/
-    cp -R $out/lib/perl5/5* $out/lib/perl5/site_perl
-  '';
 }