ansible,jenkins: rm centos7-release-sources host

[targos]

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

TODO:

• Remove from secrets repo
• Delete DO droplet
• Delete agent from Jenkins

[richardlau]

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

build/README.md

Lines 294 to 296 in 2e7c763

	The Build WG will keep build configuration required for a release line for 6
	months after the release goes End-of-Life, in case further build or test runs
	are required. After that the configuration will be removed.

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