rgw: drop rgw_obj_key::to_str()

rgw: drop rgw_obj_key::to_str()

rgw_obj_key::to_str() is mainly used by operator<<(ostream&, ..), so
we can just implement it with the specialization of
fmt::formatter<rgw_obj_key>. and let operator<<(ostream&, ..) to
call into fmt::format(..):

1. for better readability and
2. for probably better performance -- we don't need to do deep copy
   for constructing a `std::string` from a `char[]`.
3. for better standard compliance -- we don't need to use variable-length
   array in C++ code. it is a part of C99 standard. but not a C++ standard.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>