Remove deprecated PluginContext type

[ybiquitous]

Which issue, if any, is this issue related to?

Ref: #6466

Is there anything in the PR that needs further explanation?

The PluginContext type has been marked as deprecated in PR #6466.

Note that the base branch of this PR is v15.

[changeset-bot]

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Latest commit: baaac34

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[ybiquitous]

[ask] Should we add a changelog entry for removing only types? I'm concerned that it will not be possible to change internal types in the future easily.