[9.20] fix: dev: don't use 'create' flag unnecessarily in findnode()

when searching the cache for a node so that we can delete an rdataset, it isn't necessary to set the 'create' flag. if the
node doesn't exist yet, we won't be able to delete anything from it anyway.

Backport of MR !9158

Merge branch 'backport-each-minor-findnode-refactor-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9253