fix: refactor '_nextcarry-outindex' to have the same signature everywhere

[ioanaif]

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/ioanaif-refactor-nextcarry-outindex/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1911 (1ede2d4) into main (3894053) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/bytemaskedarray.py	87.88% <100.00%> (-0.16%)	⬇️

[jpivarski]

I see; ByteMaskedArray._nextcarry_outindex just didn't need it before. But it's better to be consistent, even if superfluous.

Go ahead and merge, so that you can use this in #1904!