[Parquet] Remove the BIT_PACKED encoder

[alamb]

Thanks @HippoBaro, this looks even better now. I'm fine with simply removing the dead code in this PR.

This does have me wondering if we should remove the BIT_PACKED encoder. We need a decoder for backwards compatibility, but we shouldn't ever be using it for level encoding.

Originally posted by @etseidl in #9447 (comment)

[jhorstmann]

Related to #7739, I've been thinking whether we could repurpose BIT_PACKED to mean a rle-hybrid encoder which only writes one big bitpacked run. But as far as I remember, bitpacked encoding is not exposed in any higher-level api anyway, so should be fine to remove completely.

[alamb]

label_issue.py automatically added labels {'parquet'} from #9656

[alamb]

label_issue.py automatically added labels {'performance'} from #9447