mutable_db_options: fix a Clang 14 build error (#57)

Clang 14 fails to compile with the following error:

```
options/db_options.h:120:21: error: definition of implicit copy constructor for 'MutableDBOptions' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
  MutableDBOptions& operator=(const MutableDBOptions&) = default;
                    ^
db/compaction/compaction_job.cc:439:7: note: in implicit copy constructor for 'rocksdb::MutableDBOptions' first required here
      mutable_db_options_copy_(mutable_db_options),
      ^
```

This is caused by the introduction of an assignment operator in #7 due to
a diagnostic emitted by Clang 13 which isn't emitted by Clang 14, so this
was probably a compiler bug, and we just need to remove the assignment
operator.

options/db_options.h

@@ -119,8 +119,6 @@ struct MutableDBOptions {
   MutableDBOptions();
   explicit MutableDBOptions(const DBOptions& options);
 
-  MutableDBOptions& operator=(const MutableDBOptions&) = default;
-
   void Dump(Logger* log) const;
 
   int max_background_jobs;