Minor: remove StringDef usage

Reviewed By: oprisnik

Differential Revision: D22574434

fbshipit-source-id: c25af781b95238e48e35ec7900a51ad2adc3feb1

Author: defHLT

imagepipeline/src/main/java/com/facebook/imagepipeline/producers/ProducerContext.java

@@ -95,7 +95,7 @@ public interface ProducerContext {
 
   void setEncodedImageOrigin(EncodedImageOrigin encodedImageOrigin);
 
-  <E> void setExtra(@ExtraKeys String key, @Nullable E value);
+  <E> void setExtra(String key, @Nullable E value);
 
   void putExtras(@Nullable Map<String, ?> extras);