RFR: Remove --source from help message

[Maurizio Cimadamore]

On Wed, 13 Mar 2024 14:04:09 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> This last reference to the `--source` option was missed when https://github.com/openjdk/jextract/pull/186 was fixed.
> 
> Removes `--source` from the help message.

Marked as reviewed by mcimadamore (Reviewer).

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PR Review: https://git.openjdk.org/jextract/pull/226#pullrequestreview-1934386279