Revert "Changing webmenc to use libwebm"

Temporary revert.
Problems with conflicting definitions of type off_t
in MSVC builds that need resolving.

c:\Program Files (x86)\
  Microsoft Visual Studio 9.0\VC\include\wchar.h(479) :
  "error C2371: 'off_t' : redefinition; different basic types
c:\on2experimental\libvpx\tools_common.h(26) :
   see declaration of 'off_t'"

This reverts commit 92a4c591.

Change-Id: I535e40a18842a92e3e6e0b29e5fba66313010803