[9.20] chg: minor cleanup in qpcache

when the QP cache was adapted from the RBTDB, some variable names weren't changed, and still incorrectly reference the RBT.

Backport of MR !9515

Merge branch 'backport-each-qpcache-nits-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9525