jj: drop watchman dependency (but keep the feature)

[chriskrycho]

In a previous change, we added the Cargo feature for Jujutsu’s watchman integration, but since jj works fine without watchman, and watchman is a large dependency in its own right, switch it so the Jujutsu compilation feature is enabled, but the jj recipe does not depend on watchman: people who want the integration can install it themselves.

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