Remove some unnecessary extended partition checks from GParted_Core (#788308)

For example GParted_Core::read_label() is already called for empty
partitions where .filesystem = FS_UNKNOWN.  In that case get_fs()
returns an unsupported capability set with all capabilities set to
FS::NONE.  The same would be true extended partitions where .filesystem
= FS_EXTENDED too; get_fs() would return an unsupported capability set
with all capabilities set to FS::NONE.

Therefore the if not extended partition condition around the switch
statements is not necessary in GParted_Core::read_label(), read_uuid(),
label_filesystem() and change_uuid().  Remove.

This also achieves removal of some uses of partition type enumerations
in advance of the introduction of new partition type TYPE_UNPARTITIONED.

Bug 788308 - Remove whole_device partition flag