kbn-doc-links: remove functionbeat (#198902)

## Summary

Remove docs link to functionbeat as it has been deprecated and will be
removed in 9.0

### Checklist

Delete any items that are not applicable to this PR.


### Risk Matrix

Delete this section if it is not applicable to this PR.

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#_add_your_labels)
- [ ] This will appear in the **Release Notes** and follow the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

### Related Issues:
 - Closes #193030
 - Relates elastic/beats#40745

packages/kbn-doc-links/src/get_doc_links.ts

@@ -287,9 +287,6 @@ export const getDocLinks = ({ kibanaBranch, buildFlavor }: GetDocLinkOptions): D
       base: `${ELASTIC_WEBSITE_URL}guide/en/logstash/${DOC_LINK_VERSION}`,
       inputElasticAgent: `${ELASTIC_WEBSITE_URL}guide/en/logstash/${DOC_LINK_VERSION}/plugins-inputs-elastic_agent.html`,
     },
-    functionbeat: {
-      base: `${ELASTIC_WEBSITE_URL}guide/en/beats/functionbeat/${DOC_LINK_VERSION}`,
-    },
     winlogbeat: {
       base: `${ELASTIC_WEBSITE_URL}guide/en/beats/winlogbeat/${DOC_LINK_VERSION}`,
     },

packages/kbn-doc-links/src/types.ts

@@ -243,9 +243,6 @@ export interface DocLinks {
     readonly base: string;
     readonly inputElasticAgent: string;
   };
-  readonly functionbeat: {
-    readonly base: string;
-  };
   readonly winlogbeat: {
     readonly base: string;
   };