lua: get rid of knot_rrset_txt_dump

I believe it was unused, so no risk was caused, but better remove it.
There's the complication that its signature has changed since libknot-2.4.0.
Fixes https://gitlab.labs.nic.cz/knot/resolver/issues/146
3 jobs for lua-clear-dump in 1 minute and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #2006
docker linux amd64
build:linux:amd64

00:00:28

 
  Test
passed #2008
docker linux amd64
deckard:linux:amd64

00:00:55

passed #2007
docker linux amd64
test:linux:amd64

00:00:22