pack-bitmap: drop `_1` suffix from functions that open bitmaps

pack-bitmap: drop `_1` suffix from functions that open bitmaps

In the preceding commit we've refactored how we open bitmaps. As part of
the refactoring we have consolidated `open_pack_bitmap()` as well as
`open_midx_bitmap()` into `open_bitmap_for_source()`. Consequently, we
only have their `open_pack_bitmap_1()` and `open_midx_bitmap_1()`
variants left over, where the `_1` suffix doesn't really make much sense
anymore.

Drop the suffix.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>