seastar::format() just forward the parameters to be formatted to
`fmt::format_to()`, which is able to format `std::string`, so there is
no need to cast the `std::string` instance to `std::string_view` for
formatting it.
in this change, the cast is dropped. simpler this way.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#15143