Remove redundant check in PbMap equality check (#604)

Since we check that both maps have same number of keys, the check
comparing each key in one of the maps with the value of the key in the
other map is enough to make sure both maps have the same set of keys.

protobuf/lib/src/protobuf/pb_map.dart

diff --git a/protobuf/lib/src/protobuf/pb_map.dart b/protobuf/lib/src/protobuf/pb_map.dart
index 726ead6..5c2c28c 100644
--- a/protobuf/lib/src/protobuf/pb_map.dart
+++ b/protobuf/lib/src/protobuf/pb_map.dart
@@ -52,11 +52,6 @@
       return false;
     }
     for (final key in keys) {
-      if (!other.containsKey(key)) {
-        return false;
-      }
-    }
-    for (final key in keys) {
       if (other[key] != this[key]) {
         return false;
       }