C++ standard enforces copy elision in this case. and copy elision is
more performant than constructing the return value with a move
constructor, so no need to use `std:move()` here.
and GCC-14 rightfully points this out:
```
/home/kefu/dev/scylladb/lang/lua.cc: In member function ‘data_value {anonymous}::from_lua_visitor::operator()(const utf8_type_impl&)’:
/var/ssd/scylladb/lang/lua.cc:797:25: error: redundant move in return statement [-Werror=redundant-move]
797 | return std::move(s);
| ~~~~~~~~~^~~
/home/kefu/dev/scylladb/lang/lua.cc:797:25: note: remove ‘std::move’ call
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#19187