chore: removes unused introspector-gadget crate

[EverlastingBugstopper]

This PR removes the introspector-gadget dependency from the Cargo.toml as I don't believe it's used by this crate anymore. If this passes I think I'd like to deprecate that crate and move it fully back into the Rover repository so it's easier to make changes to its implementation.

[router-perf]

CI performance tests

• step - Basic stress test that steps up the number of users over time
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• const - Basic stress test that runs with a constant number of users
• reload - Reload test over a long period of time at a constant rate of users
• large-request - Stress test with a 1 MB request payload
• events - Stress test for events with a lot of users and deduplication ENABLED
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• no-graphos - Basic stress test, no GraphOS.

[github-actions]

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