fix: dev: remove the rndc testgen command

testgen existed only to let the rndc system test generate large response payloads.
It accepted an unbounded count and was reachable from read-only control channels,
so any read-only rndc client could drive named into memory exhaustion. The command
and its supporting test helper are gone; remaining rndc commands already produce
non-trivial responses, so transport coverage is preserved.

Closes #5911

Merge branch '5911-rndc-testgen-32bit-truncation-memory-exhaustion' into 'main'

See merge request isc-projects/bind9!11912