Prevent restart for removed config entries

"silent_mode" has been removed by the upstream with commit
f7ea6beaf4ca02b8e6dc576255e35a5b86035cb9. "unix_socket_directory"
has been turned into "unix_socket_directories" with commit
c9b0cbe98bd783e24a8c4d8d8ac472a494b81292.

It wouldn't matter if we trigger restart for those configuration
variables, as they are not valid so cannot be declared anymore, but
why not do it as we have the infrastructure. PostgreSQL wouldn't start
when they are on the configuration file. Preventing the restart can
save people from having their database shut-down, because of
not-anymore-valid entry.

The change is also good for maintenance of the configuration variables.
We wouldn't forget to remove them after the releases are not supported
anymore.