Remove third_party/JSON

JSON.pm was needed for the Perl IDL compiler, and it was the only client.
This is now gone, so we can remove it.

BUG=347863
R=haraken
TBR=cpu

Review URL: https://codereview.chromium.org/183763019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254453 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254453