[libc++] Remove anchors for agent queues from the BuildKite pipeline configuration (#75359)

We didn't save that much boilerplate with those anchors, but it made the
pipeline description harder to understand because the definition of the
jobs and the agents they run on were so far apart. Anchors are useful
for the other common boilerplate we truly share between all jobs, but it
seems reasonable to define agent queues in-line.