refactor(parser): check_call dedup (nushell#16596)

Cleanup duplicated code and computation of `check_call` and `is_help`
checks, one of the many causes of nushell#14697 .

`check_call` is already called once in `parse_internal_call`, (re)using
its result should be good enough.

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Co-authored-by: Bahex <[email protected]>

Author: blindFS