Remove support for rsyslogd on CentOS 5.

CentOS 5 is out of support since half a year, and we don't have any
machines running that anymore, so we can safely retire that support,
making the manifest a little bit cleaner.  (Don't worry, we'll foul
it up with EL-7 support soon!)

This was originally commit 11b7ce36c077 in intserv.