remove absolute_delta_ field and syntax code

we'll always encode using absolute value, not relative ones.
    Both methods use the same number of bits, so we'll go for the
    simpler and most robust one.
    + add some extra checks about pic->u/v being NULL.

Change-Id: I98ea01a1a6b133ab3c816c0fbc50e18269bd2098

src/enc/vp8enci.h

@@ -159,9 +159,6 @@ typedef struct {
   int num_segments_;      // Actual number of segments. 1 segment only = unused.
   int update_map_;        // whether to update the segment map or not.
                           // must be 0 if there's only 1 segment.
-  int absolute_delta_;    // if true, segments' filter strength and quantizers
-                          // are not transmitted relatively to
-                          // VP8FilterHeader::level_ and base_quant_
   int size_;              // bit-cost for transmitting the segment map
 } VP8SegmentHeader;