Removal of CONFIGURE_SEGMENTATION ifdefs.

Removal of legacy support code fo the old coding mechanism.

Use local reference "xd" for MACROBLOCKD structure in
encode_frame_to_data_rate()

Moved call to choose_segmap_coding_method() out of encode
loop as the cost of segmentation is not properly accounted
in the loop anyway. If this is desirable in the future it
can be moved back. The use of this function to do all the
analysis and set the probabilities also removes the need
to track segment useage in threading code.

Change-Id: I85bc8fd63440e7176c73d26cb742698f9b70cade