Always use default RCU variant in pairwise builds

Always use default RCU variant in pairwise builds

Commit 42d43aa0758513a45b54e0fd0bff4381fdc4d803 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.