Unsupport Ubuntu 12.04 as a build environment

With this CL, Ubuntu 12.04 is no longer a reference environment to build Mozc binaries for Android, NaCl, and Linux.  Hereafter only Ubuntu 14.04 is supported to build those binaries.
This is just a removal of Docker files.  Hence no behavior change is intended.

BUG=none
TEST=none

git-svn-id: https://mozc.googlecode.com/svn/trunk@486 a6090854-d499-a067-5803-1114d4e51264

Author: yukawa

src/docker/ubuntu12.04/Dockerfile

@@ -1,6 +1,6 @@
 MAJOR=2
 MINOR=16
-BUILD=2019
+BUILD=2020
 REVISION=102
 # NACL_DICTIONARY_VERSION is the target version of the system dictionary to be
 # downloaded by NaCl Mozc.