fix: remove unused to_string_view overload in fmt >= 11.1

[jdrouhard]

This overload was added in #3301 based on a compiler error quickfix suggestion.

fmt's fmt::format_string type has had an implicit conversion operator to fmt::string_view since fmt 8.0 (introduced here).

Since there is already an overload for to_string_view() that no-ops spdlog::string_view_t (which is typedef'd to fmt::string_view when using fmt), we don't need any other overloads for format_string.