In this PR, we get rid of the unnecessary default switch case in `prefix_for_scheme()`. We also change the return type of the function to `std::string_view` as it's easier to operate on.
Backport: not needed; this is a code cleanup.
Closesscylladb/scylladb#21749
* github.com:scylladb/scylladb:
auth/passwords: Change return type of prefix_for_scheme to std::string_view
auth/passwords.cc: Remove default case in prefix_for_scheme()