vinyl: remove pointless is_nullable initialization for disk_format

space->format and cmp_def must be compatible, i.e. space->format has
is_nullable flag set for a field iff it is set for all key parts
indexing this field. Therefore there's no point to set is_nullable for
disk_format as it must have been initialized by tuple_format_create().
Remove the pointless loop.

Also, while we are at it, fix the minor memory leak - disk_format is
referenced twice for the primary key.