CLN: commented-out code in _libs/sparse.pyx

[jbrockmendel]

reindex_block, SparseCruncher, and (related) reindex_integer

[jbrockmendel]

BlockIntersection

get_reindexer appears unused

[gfyoung]

I'm all for removing commented-out code, since if there was a use for it, it would have been commented-in already awhile ago.

cc @jreback

[jbrockmendel]

Closed by #23583