utils: chunked_vector: Do not require T to be default-constructible for clear()
utils: chunked_vector: Do not require T to be default-constructible for clear()

resize(), used by clear(), requires T to be default-constructible in
case the vector is expanded. It's not actually needed for clearing,
and there will be users which use clear() with
non-default-constructible T, so implement clear() without using
resize().

avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla