This is a slightly different split in how to tackle bug 1748222.
Instead of removing Resource API function one by one, here, I'm migrating the whole sources reducer+selector to a JS Map.

This makes the patch slightly bigger, but this is probably mandatory step toward migrating to a Map.

It looks like there is no major regression while migrating to a map:
https://treeherder.mozilla.org/perfherder/comparesubtest?originalProject=try&newProject=try&newRevision=146f3014cdcf000eb05d889afdbe664943bfb1e0&originalSignature=3130994&newSignature=3130994&framework=12&originalRevision=5c95eebb4a5814f97bd991389557a5201859d0d6&page=1&showOnlyConfident=1
That, even if the current patch does copy the whole map on each modification/addition/removal!
We can probably followup and have a mutable map in the reducer.

try run:
https://treeherder.mozilla.org/jobs?repo=try&revision=ef9d94e34388a97a46c75eecb3dd252bffa9a022

https://hg.mozilla.org/mozilla-central/rev/7a205bdbd2e2