mass-delta-builder: Remove unused enum values

mass-delta-builder: Remove unused enum values

In the enum "DeltaType", there were values named "kernel", "system", but
they are not used. (DeltaType was checked only if it is "full" or not)
These unused enum values are removed.

Also their values are defined with 'enum.auto()' function instead of
integer value since it is better to distinguish enum elements when the
new element is added or element is removed.

Change-Id: I607c9358f6c03ae4a333d21705c0775d731b1b10
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>