Strip '-Werror' from build.xml in the public version of Buck.

Summary:
From https://github.com/facebook/buck/pull/16.

This reportedly causes problems when building Buck with OpenJDK 7:
https://github.com/spearce/buck/commit/243470c3c124af0da67148e3fa6e3efcf440d597.

Admittedly, we do not officially support the OpenJDK, but if it's not
hard to support it, then we should at least try.

Test Plan: Ran `./opensource/create_codebase --local` and verified the scrubbing worked.
diff --git a/build.xml b/build.xml
index 498dd53..a8748ff 100644
--- a/build.xml
+++ b/build.xml
@@ -93,7 +93,6 @@
            >
       <compilerarg value="-Xlint:rawtypes" />
       <compilerarg value="-Xlint:unchecked" />
-      <compilerarg value="-Werror" />
     </javac>
   </target>
 
@@ -106,7 +105,6 @@
            >
       <compilerarg value="-Xlint:rawtypes" />
       <compilerarg value="-Xlint:unchecked" />
-      <compilerarg value="-Werror" />
     </javac>
   </target>
 
@@ -127,7 +125,6 @@
       <exclude name="com/facebook/buck/java/abi/**" />
       <compilerarg value="-Xlint:rawtypes" />
       <compilerarg value="-Xlint:unchecked" />
-      <compilerarg value="-Werror" />
     </javac>
   </target>
 
@@ -178,7 +175,6 @@
            >
       <compilerarg value="-Xlint:rawtypes" />
       <compilerarg value="-Xlint:unchecked" />
-      <compilerarg value="-Werror" />
     </javac>
   </target>