Remove contrib/externs/angular.js from Closure Compiler.

The AngularJS externs files have been moved to the AngularJS project itself, into closure/angular.js. This is to make it easier to maintain externs files that match the AngularJS version's API, and get the definitions closer to their actual source.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=56102082
Add unusedPrivateMembers as a valid @suppress annotation.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=56119069