block: Clean up some bad code in the vvfat driver

Remove the unnecessary home-grown redefinition of the assert() macro here,
and remove the unusable debug code at the end of the checkpoint() function.
The code there uses assert() with side-effects (assignment to the "mapping"
variable), which should be avoided. Looking more closely, it seems as it is
apparently also only usable for one certain directory layout (with a file
named USB.H in it) and thus is of no use for the rest of the world.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
