Always use default RCU variant in pairwise builds

Commit 42d43aa0 made --with-liburcu depend on --enable-developer. This broke pairwise testing as this new dependency was not codified in configure.ac. Since the --with-liburcu option is currently just a convenience for developers, there is no need to test building against all possible RCU variants in GitLab CI until they actually work with BIND 9. Update the pairwise testing "configuration" in configure.ac so that builds with non-standard RCU variants are not tested.

See https://gitlab.isc.org/isc-projects/bind9/-/jobs/3756242 for a sample failure that this MR intends to "fix".

Pipeline for this MR that includes the pairwise job:

https://gitlab.isc.org/isc-projects/bind9/-/pipelines/151585

Mattermost discussion:

https://mattermost.isc.org/isc/pl/6p9sxd58xp8f7q1qyoc1ktjyee

Closes #4390 (closed)