fix(python): Remove incorrect last byte zeroing in Python buffer construction

[paleolimbot]

There was some code that zeroed out the last byte of a validity buffer in the Python buffer construction from an iterable; however, we already zero out the last byte when reserving/allocating an ArrowBitmap.

Checked locally with:

export NANOARROW_PLATFORM=ubuntu 
export NANOARROW_ARCH=arm64
docker compose run --rm -e TEST_WITH_MEMCHECK=1 -e TEST_DEFAULT=0 -e TEST_PYTHON=1 verify

Closes #821.