Remove Vulnerability Post Processing code for CentOS

[lucasmrod]

Once OVAL is used for processing CentOS hosts we should remove all the "Post Processing" code:

Code to remove:

• https://github.com/fleetdm/fleet/blob/main/cmd/fleet/cron.go#L276-L284
• https://github.com/fleetdm/fleet/blob/main/server/vulnerabilities/centos.go
• https://github.com/fleetdm/fleet/blob/main/server/vulnerabilities/vuln_centos

[lucasmrod]

Fixed in #6241.