DMU API: Removed obsolete element property identifiers

The property identifiers DMU_FRONT_SECTOR and DMU_BACK_SECTOR are
now obsolete as these properties can now always be addressed using
the relative accessor flags DMU_FRONT_OF_LINE, DMU_BACK_OF_LINE.

DMU_FRONT_SECTOR and DMU_BACK_SECTOR have been moved to dmu_lib.h
and are now implemented as short-hand aliases.