pack-bitmap: clarify comment in filter_bitmap_exclude_type()

pack-bitmap: clarify comment in filter_bitmap_exclude_type()

The code that eventually became filter_bitmap_exclude_type() was
originally introduced in 4f3bd5606a (pack-bitmap: implement BLOB_NONE
filtering, 2020-02-14) to accelerate BLOB_NONE filters with bitmaps.

In 856e12c18a (pack-bitmap.c: make object filtering functions generic,
2020-05-04), it became filter_bitmap_exclude_type(). But not all of the
comments were updated to be agnostic to the provided type.

Remove the remaining comments which should have been updated in
856e12c18a to reflect the type-agnostic nature of the function.

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