Remove old *::Clusters::IntegrationsController classes (dead code)

The following discussion from !137829 (merged) should be addressed:

!137829 (comment 1685166426):

Oh right, I'm pretty sure the *::Clusters::IntegrationsController classes are deprecated / no longer being "hit". This was introduced for the Prometheus integration through the certificate-based cluster, and there were no changes around this area since.

Looks like the UI was removed completely in !122043 (merged). So yeah, this is dead code.

/cc @partiaga @hustewart @nagyv-gitlab