[C++][Parquet] Simplify `BitReader::GetVlqInt`

[pitrou]

Describe the enhancement requested

The BitReader::GetVlqInt implementation currently tries to read first from the cached value before falling back to reading from the buffer.

But this doesn't bring any benefit, since both code paths lead to the same processing step afterwards. So we can remove the code path that tries to read from the cached value. This will also make it easier to support big-endian platforms.

Component(s)

C++, Parquet

[pitrou]

Issue resolved by pull request 48237
#48237