this is a part of a series to migrating from `operator<<(ostream&, ..)` based formatting to fmtlib based formatting. the goal here is to enable fmtlib to print `function_name` without the help of `operator<<`. the corresponding `operator<<()` are dropped dropped in this change, as all its callers are now using fmtlib for formatting now. Refs #13245 Signed-off-by: Kefu Chai <kefu.chai@scylladb.com> Closes #13608