Quit use bigram probability diff for ver4 dict.

java/src/com/android/inputmethod/latin/BinaryDictionary.java

...ve/jni/com_android_inputmethod_latin_BinaryDictionary.cpp

...l/dictionary/structure/v4/content/bigram_dict_content.cpp

...mpl/dictionary/structure/v4/content/bigram_dict_content.h

...mpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp

.../com/android/inputmethod/latin/BinaryDictionaryTests.java