Remove legacy CI component fetching

Context

In #415853 (closed), we made it possible to fetch components from component repositories using the new directory structure. We have supported both the new and legacy fetching for some time and we now want to stop supporting the legacy fetching.

Proposed solution

• Remove include:component fetching for legacy component paths

Notes:

• The specs for Ci::Components::FetchService must be updated - see note here: !130214 (comment 1533504986)

Please consider: !127448 (comment 1505345297):

we should reconsider whether it makes sense to still pass in content_filename: here (currently its potential value is only template.yml). Perhaps we should pass in the top-level directory name (i.e. "templates") instead and then define the default name template.yml if the directory is "templates"