because its base class of `writer_impl` has a member variable
`_validator`, which has its copy ctor deleted. let's just
drop the defaulted move ctor, as compiler is not able to
generate one for us.
```
/home/kefu/dev/scylladb/sstables/mx/writer.cc:805:5: error: explicitly defaulted move constructor is implicitly deleted [-Werror,-Wdefaulted-function-deleted]
writer(writer&& o) = default;
^
/home/kefu/dev/scylladb/sstables/mx/writer.cc:528:16: note: move constructor of 'writer' is implicitly deleted because base class 'sstable_writer::writer_impl' has a deleted move constructor
class writer : public sstable_writer::writer_impl {
^
/home/kefu/dev/scylladb/sstables/writer_impl.hh:29:48: note: copy constructor of 'writer_impl' is implicitly deleted because field '_validator' has a deleted copy constructor
mutation_fragment_stream_validating_filter _validator;
^
/home/kefu/dev/scylladb/mutation/mutation_fragment_stream_validator.hh:188:5: note: 'mutation_fragment_stream_validating_filter' has been explicitly marked deleted here
mutation_fragment_stream_validating_filter(const mutation_fragment_stream_validating_filter&) = delete;
^
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#12877
45f0449ccf