sstring: drop partial specialization for std container types
sstring: drop partial specialization for std container types

std::vector and std::unordered_map, as well as fmt::formatter, are not
seastar types, so seastar has no business defining these specializations.

Instead, applications should do that, or (better) use <fmt/ranges.h> or
fmt::join.

scylladb/seastar,syuu1228/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar