Re-evaluate skipping the chained fast backend in the installer

Problem/Motivation

Spin-off from #3583040: Cache the container definition in APCu.

We have the following in ChainedFastBackendFactory

    // Do not use the fast chained backend during installation. In those cases,
    // we expect many cache invalidations and writes, the fast chained cache
    // backend performs badly in such a scenario.
    if (!InstallerKernel::installationAttempted()) {
      $this->fastServiceName = $fast_service_name;
    }

This is no longer the case since #3526080: Reduce write contention to the fast and consistent backend in ChainedFastBackend.

I don't think we'll see a performance improvement, because the hit rate would still be extremely low, but the cache stampede issue should be fixed, so we could remove a small amount of code.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet