Today marks the official end of the 2023 OpenInfra Summit CFP, so we
can remove the banner promoting it. This mostly reverts
f5e065fe71785ad778233e36a19d77025057438f but leaves some of the CSS
in place in order to support easier addition and removal of banner
content in the future. Ideally we'd find a balance where we don't
need further CSS edits, but...
Change-Id: I2295eac28ce4e2e47ad8f3a280dd6a4c2e41f849