src: remove validation of unreachable code

Based on https://github.com/nodejs/help/issues/2600#issuecomment-612857153
the condition (amount < 0) won't be possible.

PR-URL: https://github.com/nodejs/node/pull/32818


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
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