Removes ignored const from return type (#2915)

See https://github.com/google/protobuf/issues/2425

src/google/protobuf/map.h

diff --git a/src/google/protobuf/map.h b/src/google/protobuf/map.h
index e6d78b2..88878b5 100644
--- a/src/google/protobuf/map.h
+++ b/src/google/protobuf/map.h
@@ -678,7 +678,7 @@
 
     const Key& key() const { return k_; }
     Key& key() { return k_; }
-    value_type* const value() const { return v_; }
+    value_type* value() const { return v_; }
     value_type*& value() { return v_; }
 
    private: