CLN: remove BlockManager.get
This should not be merged until after #33047 and #33045, since the assertions made here will not always hold until then.
Looks good - any perf implications? > any perf implications?

_getitem_multilevel calling _ixs instead of _get_item_cache is slightly higher-level, and we lost a little ground on the columns.is_unique assertions, then make up some ground with unnecessary checks in BlockManager.get being removed.  It's about a wash. Updated so as to not be blocked by #33047.