because transport/server.cc has the complete definition of event_notifier, the
compiler can default-generate the destructor of `cql_server` with the necessary
information. otherwise, clang-19 would fail to build, like:
```
FAILED: CMakeFiles/scylla.dir/Dev/main.cc.o
/home/kefu/.local/bin/clang++ -DBOOST_PROGRAM_OPTIONS_DYN_LINK -DBOOST_PROGRAM_OPTIONS_NO_LIB -DDEVEL -DFMT_SHARED -DSCYLLA_BUILD_MODE=dev -DSCYLLA_ENABLE_ERROR_INJECTION -DSEASTAR_API_LEVEL=7 -DSEASTAR_ENABLE_ALLOC_FAILURE_INJECTION -DSEASTAR_LOGGER_COMPILE_TIME_FMT -DSEASTAR_LOGGER_TYPE_STDOUT -DSEASTAR_SCHEDULING_GROUPS_COUNT=16 -DSEASTAR_SSTRING -DSEASTAR_TYPE_ERASE_MORE -DXXH_PRIVATE_API -DCMAKE_INTDIR=\"Dev\" -I/home/kefu/dev/scylladb -I/home/kefu/dev/scylladb/build/gen -I/home/kefu/dev/scylladb/seastar/include -I/home/kefu/dev/scylladb/build/seastar/gen/include -I/home/kefu/dev/scylladb/build/seastar/gen/src -I/home/kefu/dev/scylladb/build -isystem /home/kefu/dev/scylladb/build/rust -isystem /home/kefu/dev/scylladb/abseil -O2 -std=gnu++23 -fvisibility=hidden -Wall -Werror -Wextra -Wno-error=deprecated-declarations -Wimplicit-fallthrough -Wno-c++11-narrowing -Wno-deprecated-copy -Wno-mismatched-tags -Wno-missing-field-initializers -Wno-overloaded-virtual -Wno-unsupported-friend -Wno-enum-constexpr-conversion -Wno-unused-parameter -ffile-prefix-map=/home/kefu/dev/scylladb=. -march=westmere -U_FORTIFY_SOURCE -Werror=unused-result -fstack-clash-protection -MD -MT CMakeFiles/scylla.dir/Dev/main.cc.o -MF CMakeFiles/scylla.dir/Dev/main.cc.o.d -o CMakeFiles/scylla.dir/Dev/main.cc.o -c /home/kefu/dev/scylladb/main.cc
In file included from /home/kefu/dev/scylladb/main.cc:11:
In file included from /usr/include/yaml-cpp/yaml.h:10:
In file included from /usr/include/yaml-cpp/parser.h:11:
In file included from /usr/lib/gcc/x86_64-redhat-linux/14/../../../../include/c++/14/memory:78:
/usr/lib/gcc/x86_64-redhat-linux/14/../../../../include/c++/14/bits/unique_ptr.h:91:16: error: invalid application of 'sizeof' to an incomplete type 'cql_transport::cql_server::event_notifier'
91 | static_assert(sizeof(_Tp)>0,
| ^~~~~~~~~~~
/usr/lib/gcc/x86_64-redhat-linux/14/../../../../include/c++/14/bits/unique_ptr.h:398:4: note: in instantiation of member function 'std::default_delete<cql_transport::cql_server::event_notifier>::operator()' requested here
398 | get_deleter()(std::move(__ptr));
| ^
/home/kefu/dev/scylladb/transport/server.hh:135:7: note: in instantiation of member function 'std::unique_ptr<cql_transport::cql_server::event_notifier>::~unique_ptr' requested here
135 | class cql_server : public seastar::peering_sharded_service<cql_server>, public generic_server::server {
| ^
/home/kefu/dev/scylladb/transport/server.hh:135:7: note: in implicit destructor for 'cql_transport::cql_server' first required here
/home/kefu/dev/scylladb/transport/server.hh:149:11: note: forward declaration of 'cql_transport::cql_server::event_notifier'
149 | class event_notifier;
| ^
1 error generated.
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
7e4e685964