doc: remove --expose-gc flag from CLI documentation

[dario-piotrowicz]

The CLI documentation for --expose-gc has been added in #53078

However it's removal was requested in #53078 (comment) (since this is not something users should generally rely on), but the documentation wasn't fully removed from the PR before it landed.

So I believe that the flag being documented might have been an oversight.

Original context: #58878 (comment)

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Note: I'm trying to figure out here if --expose-gc should or not be documented in the CLI documentation, and if it should be I would be inclined to also include it in the manpage for consistency 🤔

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Note: I wanted to also mention that the flag is also included in the allowed v8 options and the useful v8 options
(meaning that even after removing the cli flag section some references to it, for advanced users, will still be present in the document)

[jakecastelli]

This one feels similar to the --expose-internals CLI flag which exists but not documented.

We use --expose-internals exclusively in Core for various testing purposes but outside of core development it is not encouraged.