paritionedfs: remove the 'extended' key from disk dictionary

paritionedfs: remove the 'extended' key from disk dictionary

The 'extended' key holds the size of the extended partition, and it is only
used once when we create the extended partition. However, we do not really need
this becase we can easily calculate the extended partition size, because we
know the start of the first logical partition and the disk size. Having this
'extended' key only complicates the code unnecessarily.

Change-Id: I58e61e219890339fae63eeb5bdf9f2002401cb72
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>