Removed references to the deprecated Collector

[ekansa]

brief description of changes

Removed reference (except for a historical note) about Collector. The GCI Arches team requested this to avoid confusion about the status of Collector.

issues addressed

(https://github.com/archesproject/arches-docs/issues/284)

further comments

The v6 branches have a deprecation warning, the v7 branches (and with this PR the master branch) will have all reference to Collector purged.

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This box must be checked

• [X] the PR branch was originally made from the base branch

This box should be checked

• [X] after these changes the docs build locally without error

This box should only be checked you intend to follow through on it (we can do it on our end too)

• [ ] I will cherry-pick all commits in this PR into other branches that should have them after this PR is merged

[ekansa]

Sorry to bug you with another PR today @mradamcox, but since this is for the master branch, I think it's a good practice to make sure someone looks over my shoulder.