Revert r438 "Update Protocol Buffers to 2.6.1" and r444 "Suppress compiler warnings for Protocol Buffers 2.6.1"

Although this CL breaks aarch64/mipsel64 build in Android, we should fix Windows test first.

Closes Issue 271.

BUG=Issue mozc:271
TEST=unittest on Windows 7

git-svn-id: https://mozc.googlecode.com/svn/trunk@482 a6090854-d499-a067-5803-1114d4e51264