Remove third_party/nss_pkcs12 from the repository.

nss_pkcs12/README

Index: nss_pkcs12/README
===================================================================
--- nss_pkcs12/README	(revision 26000)
+++ nss_pkcs12/README	(working copy)
@@ -1,18 +0,0 @@
-Name: Network Security Services (NSS) pkcs12 directory
-Short Name: nss_pkcs12
-URL: http://www.mozilla.org/projects/security/pki/nss/
-Version: 3.15.1
-License: MPL 2
-License File: COPYING
-
-Description:
-This is a copy of the directory nss/lib/pkcs12 from the NSS open-source
-secure networking library. Dart gets the rest of NSS from Chromium's
-modified copy of NSS, but Chromium's version omits this directory.
-
-This has been copied from a downloaded copy of NSS, rather than checked out
-from the original repository. No changes or patches have been made to it.
-This directory should be updated manually whenever the version of NSS in
-Dart's checkout of third_party/nss from Chromium is updated.
-
-The file COPYING has been copied from nss/COPYING in the original.