qcow2: Don't check nb_clusters when removing l2meta from the list

In the past, when a new cluster was allocated the l2meta structure was
a variable in the stack so it was necessary to have a way to tell
whether it had been initialized and contained valid data or not. The
nb_clusters field was used for this purpose. Since commit f50f88b9
this is no longer the case, l2meta (nowadays a pointer to a list) is
only allocated when needed and nb_clusters is guaranteed to be > 0 so
this check is unnecessary.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <ab0b67c29c7ba26e598db35f12aa5ab5982539c1.1599150873.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>