Core: remove ASSIGNED_TRIP trip flag

The distinction between ASSIGNED_TRIP and IN_TRIP was used to
prefer non-autogenerated trips on merging of dives. But owing
to bit rot this seem to have worked only partially anyway:
The IN_TRIP field was set in create_and_hookup_trip_from_dive()
and immediately overwritten in add_dive_to_trip() called
in the next line.

Instead, use the trip->autogen flag to check for priority and
remove the ASSIGNED_TRIP flag alltogether.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/dive.h

@@ -282,7 +282,6 @@ typedef enum {
 	TF_NONE,
 	NO_TRIP,
 	IN_TRIP,
-	ASSIGNED_TRIP,
 	NUM_TRIPFLAGS
 } tripflag_t;