ansible,jenkins: rm centos7-release-sources host

[targos]

trafficstars

Since v16 went EoL, RHEL8 is used for all release lines.

TODO:

• Remove from secrets repo
• Delete DO droplet

[richardlau]

It's written down that we keep stuff around for 6 months after a release line goes End-of-Life before removing.

https://github.com/nodejs/build/blob/2e7c763cc6b02eee6edbe1f5199a54350b273e2a/README.md?plain=1#L294-L296

FWIW I can only remember one time when we decided to do an additional release after End-of-Life, which IIRC was within the month after the End-of-Life.

[targos]

We're more than 6 months later now