crimson/osd: fix omap key removal encoding mismatch

crimson/osd: fix omap key removal encoding mismatch

RGW bucket deletion via cls_user_remove_bucket was failing on Crimson
because cls_cxx_map_remove_key() encoded a std::vector<string> while
PGBackend::omap_remove_key() expected and decoded a std::set<string>
using decode_str_set_to_bl(). This mismatch caused OMAP keys to not be
removed, leaving stale bucket entries that broke test_list_buckets_*.

Fix by encoding std::set<std::string> in cls_cxx_map_remove_key() to
match the backend's decode_str_set_to_bl() expectation, aligning with
classic OSD behavior.

This fixes: s3 test_list_buckets_* tests on Crimson

Signed-off-by: Kautilya Tripathi <kautilya.tripathi@ibm.com>