block: remove QED .bdrv_make_empty implementation

The QED .bdrv_make_empty() implementation does nothing but return
-ENOTSUP, which causes problems in bdrv_commit().  Since the function
stub exists for QED, it is called, which then always returns an error.

The proper way to not support an optional driver function stub is to
just not implement it, so let's remove the stub.
block: remove qcow2 .bdrv_make_empty implementation

The QCOW2 .bdrv_make_empty implementation always returns 0 for success,
but does not actually do anything.

The proper way to not support an optional driver function stub is to
just not implement it, so let's remove the stub.