(RE-2368) Update for CentOS 7

We can now remove the bandaids because CentOS 7 has been officially
released. This consists of removing the bandaid files themselves and
updating the mocks to remove lingering patch code. We also have updated
the pe-el-mock-config template to pull from osmirror instead of yo.
See the comment I left for `manifests/mock/pe_mockset.pp`, about how this platform isn't actually a thing.