The upstream pg_cancel_backend() function should work fine on GPDB. It's
not exactly the same: in gp_cancel_query(), you would pass the sesssion ID
and command ID as argument, while pg_cancel_backend() takes a PID. But it
seems just as good from a usability point of view. Let's avoid the
duplicate code, and remove gp_cancel_query().

Also remove the gp_cancel_query_print_log and gp_cancel_query_delay_time
GUCs. They were not directly related to gp_cancel_query() - they would
have an effect on cancellations caused by pg_cancel_backend() or
statement_timeout, too. But they were marked as "developer options", and
they printed the session and command ID so I think they were meant to be
used with gp_cancel_backend(). I don't think anyone uses them, though, so
let's just remove them, rather than change them to print PIDs.