t: retire 'GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP'

Two years ago, commit ff1e653c8e (midx: respect
'GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP', 2021-08-31) introduced a new
environment variable which caused the test suite to write MIDX bitmaps
after any 'git repack' invocation.

At the time, this was done to help flush out any bugs with MIDX bitmaps
that weren't explicitly covered in the t5326-multi-pack-bitmap.sh
script.

Two years later, that flag has served us well and is no longer providing
meaningful coverage, as the script in t5326 has matured substantially
and covers many more interesting cases than it did back when ff1e653c8e
was originally written.

Remove the 'GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP' environment variable
as it is no longer serving a useful purpose. More importantly, removing
this variable clears the way for us to introduce a new one to help
similarly flush out bugs related to incremental MIDX chains.

Because these incremental MIDX chains are (for now) incompatible with
MIDX bitmaps, we cannot have both.

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

midx.h

@@ -29,8 +29,6 @@ struct bitmapped_pack;
 #define MIDX_LARGE_OFFSET_NEEDED 0x80000000
 
 #define GIT_TEST_MULTI_PACK_INDEX "GIT_TEST_MULTI_PACK_INDEX"
-#define GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP \
-	"GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP"
 
 struct multi_pack_index {
 	struct multi_pack_index *next;