block/write-threshold: drop extra APIs

block/write-threshold: drop extra APIs

bdrv_write_threshold_exceeded() is unused.

bdrv_write_threshold_is_set() is used only to double check the value of
bs->write_threshold_offset in tests. No real sense in it (both tests do
check real value with help of bdrv_write_threshold_get())

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210506090621.11848-5-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[mreitz: Adjusted commit message as per Eric's suggestion]
Signed-off-by: Max Reitz <mreitz@redhat.com>