Remove WPImageSource and OHHTTPStubs dependency

[kean]

I removed the last WPImageSource usage from WordPress-iOS a few months ago – it's now down to only a couple of images loading systems, down from 10+.

WordPressShared now has no dependencies except for the ones used by CI:

    dependencies: [
        .package(url: "https://github.com/buildkite/test-collector-swift", from: "0.3.0"),
        // See https://github.com/erikdoe/ocmock/issues/500#issuecomment-1002700625
        .package(url: "https://github.com/realm/SwiftLint", exact: loadSwiftLintVersion())
    ],

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• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.