[APM] Remove old get-started and server docs

[bmorelli25]

elastic/docs#2309 incorrectly moved APM docs from elastic.co/guide/en/apm/* to elastic.co/guide/*. This was fixed in elastic/docs#2324. built-docs doesn't automatically pick up this change, so the files at elastic.co/guide need to be deleted manually. This PR deletes all files in html/server/* and html/get-started/*. These files correspond to the APM docs here and here.

[dedemorton]

Only one obstacle I can think of here: is it possible that there are links to the old path (from Kibana or other docs)? If so, when you remove these files, you'll break the doc build. That seems very unlikely, though, since the new path was removed fairly quickly and our shared attributes file takes care of paths. I'll approve and you can decide what to do. I think merge it and we can always revert then clean up links, in the unlikely event that they break.

[bmorelli25]

That's a good point. I think we're safe, but I'll revert if something breaks. I'm optimistic we'll be OK since these files haven't existed for long.