block: Clean up bdrv_flush_all()

Change (!bdrv_is_removable(bs) || bdrv_is_inserted(bs)) to just
bdrv_is_inserted().  Rationale:

    The value of bdrv_is_removable(bs) matters only when
    bdrv_is_inserted(bs) is false.

    bdrv_is_inserted(bs) is true when bs is open (bs->drv != NULL) and
    not an empty host drive (CD-ROM or floppy).

    Therefore, bdrv_is_removable(bs) matters only when:

    1. bs is not open
       old: may call bdrv_flush(bs), which does nothing
       new: won't call

    2. bs is an empty host drive
       old: may call bdrv_flush(bs), which calls driver method
            raw_flush(), which calls fdatasync() or equivalent, which
            can't do anything useful while the drive is empty
       new: won't call

Result is bs->drv && !bdrv_is_read_only(bs) && bdrv_is_inserted(bs).
bdrv_is_inserted(bs) implies bs->drv.  Drop the redundant test.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -1753,8 +1753,7 @@ void bdrv_flush_all(void)
     BlockDriverState *bs;
 
     QTAILQ_FOREACH(bs, &bdrv_states, list) {
-        if (bs->drv && !bdrv_is_read_only(bs) &&
-            (!bdrv_is_removable(bs) || bdrv_is_inserted(bs))) {
+        if (!bdrv_is_read_only(bs) && bdrv_is_inserted(bs)) {
             bdrv_flush(bs);
         }
     }