Remove labels fetching for Channels in useBaseSearch

[nucleogenesis]

Overview

Follow-up to #12759 where a decision was made to remove the "Channels" label/option from the SearchFiltersPanel.

With this change, we no longer need to be fetching/processing the labels for this code so it can be removed from useBaseSearch.

Starting from around here:

https://github.com/learningequality/kolibri/blob/458bd6032e918f3b1405a40f83df5893fb7ffaaa/packages/kolibri-common/composables/useBaseSearch.js#L157

and through the file dealing with channels-related things like:

https://github.com/learningequality/kolibri/blob/458bd6032e918f3b1405a40f83df5893fb7ffaaa/packages/kolibri-common/composables/useBaseSearch.js#L236

and any other reference to channels labels.

Acceptance criteria

[Then, more detail. Provide a short summary and/or list key outcomes that the user should be able to accomplish with this feature.]

• [ ] useBaseSearch no longer exposes availableChannels
• [ ] useBaseSearch no longer processes anything to do with channels labels
• [ ] useBaseSearch still however filters search results by whatever channel the user is currently on (test only in Learn until #12819 is closed)