Revert "third_party/protobuf: Update to HEAD (83d681ee2c)"

third_party/protobuf/java/compatibility_tests/README.md

-# Protobuf Java Compatibility Tests
-
-This directory contains tests to ensure protobuf library is compatible with
-previously released versions.
-
-## Directory Layout
-
-For each released protobuf version we are testing compatibility with, there
-is a sub-directory with the following layout (take v2.5.0 as an example):
-
-  * v2.5.0
-    * test.sh
-    * pom.xml
-    * protos/ - unittest protos.
-    * more_protos/ - unittest protos that import the ones in "protos".
-    * tests/ - actual Java test classes.
-
-The testing code is extracted from regular protobuf unittests by removing:
-
-  * tests that access package private methods/classes.
-  * tests that are known to be broken by an intended behavior change (e.g., we
-    changed the parsing recursion limit from 64 to 100).
-  * all lite runtime tests.
-
-It's also divided into 3 submodule with tests depending on more_protos and
-more_protos depending on protos. This way we can test scenarios where only part
-of the dependency is upgraded to the new version.
-
-## How to Run The Tests
-
-We use a shell script to drive the test of different scenarios so the test
-will only run on unix-like environments. The script expects a few command
-line tools to be available on PATH: git, mvn, wget, grep, sed, java.
-
-Before running the tests, make sure you have already built the protoc binary
-following [the C++ installation instructions](../../src/README.md). The test
-scripts will use the built binary located at ${protobuf}/src/protoc.
-
-To start a test, simply run the test.sh script in each version directory. For
-example:
-
-    $ v2.5.0/test.sh
-
-For each version, the test script will test:
-
-  * only upgrading protos to the new version
-  * only upgrading more_protos to the new version
-
-and see whether everything builds/runs fine. Both source compatibility and
-binary compatibility will be tested.