Arch-BOM DD monitor clean-up

[robrap]

See monitors: https://app.datadoghq.com/monitors/manage?q=team%3Aarch-bom&order=desc

• SRE synthetic tests owned by SRE should not be tagged as an arch-bom alert.
  • Needs an SRE ticket to fix. A Slack post (contents below) was written in #sre-support, but it didn't get attention that way.
• Add "Safety Net - " to the start of the four edx-edxapp-lms monitors at the top. (Is this the right change to make? Are these all sustained alerts? Would the runbooks need to be updated?)
• Can the "No Data" Synthetics test monitors be deleted until they are needed?
  • Confirm with owners @jristau1984 @bszabo
• Can "Example monitor for testing time-period options" be deleted? @rayzhou-bit?

SRE support Slack post that didn't get a response:

[request] The DD monitor https://app.datadoghq.com/monitors/137916824 for the edx LMS was created by SRE and alerts SRE, but was tagged with team arch-bom. Can SRE please tag this with the SRE team instead? Thank you.