[Console] Remove an unused function for "extension" files

[yuliacech]

Summary

Related to #159041

While working on #161684, I noticed another unused function. There is no usage in the code base of the "extensions" function and I think it would be better to delete it from Console. That makes the remaining code that we need to work with easier to understand.

Release note

The function addExtensionSpecFilePath is removed from the Console plugin setup contract (server side).

[elasticmachine]

Pinging @elastic/platform-deployment-management (Team:Deployment Management)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: b8d8f92

Metrics [docs]

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	14	16	+2
securitySolution	411	415	+4
total			+6

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	15	17	+2
securitySolution	490	494	+4
total			+6

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[ElenaStoeva]

Good catch! Thanks for removing this redundancy @yuliacech. LGTM