chg: nil: Remove dns_rdataslab_merge() and friends

After the split to dns_rdataslab and dns_rdatavec, the
dns_rdataslab_merge() function was unused and it suffered from the same
data race as fixed in the previous commit.  Instead of fixing it, just
remove the function and bunch of other unused functions from the
dns_rdataslab unit.

Merge branch 'ondrej/cleanup-dns_rdataslab' into 'main'

See merge request isc-projects/bind9!11376