object-file: drop `index_blob_stream()`

object-file: drop `index_blob_stream()`

The `index_blob_stream()` function is a mere wrapper around
`index_blob_bulk_checkin()`. This has been the case since 568508e7657
(bulk-checkin: replace fast-import based implementation, 2011-10-28),
which has moved the implementation from `index_blob_stream()` (which was
still called `index_stream()`) into `index_bulk_checkin()` (which has
since been renamed to `index_blob_bulk_checkin()`).

Remove the redirection by dropping the wrapper. Move the comment to
`index_blob_bulk_checkin()` to retain its context.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>