Remove agentbeat from this repository #48145
Description
Remove https://github.com/elastic/beats/tree/main/x-pack/agentbeat and it's related build, packaging, and CI jobs as it has been merged into the new elastic-otel-collector binary introduced by elastic/elastic-agent#11821.
Agentbeat as a concept will remain in the elastic-otel-collector binary (which still has a filebeat sub-command for example) until all Beats are runnable as receivers when the beat sub-commands will be removed.