builtin/repack.c: inline `remove_redundant_bitmaps()`

builtin/repack.c: inline `remove_redundant_bitmaps()`

After writing a new MIDX, the repack command removes any bitmaps
belonging to packs which were written into the MIDX.

This is currently done in a separate function outside of
`write_midx_included_packs()`, which forces the caller to keep track of
the set of packs written into the MIDX.

Prepare to no longer require the caller to keep track of such
information by inlining the clean-up into `write_midx_included_packs()`.
Future commits will make the caller oblivious to the set of packs
included in the MIDX altogether.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>